Coordination 1

INORGANIC CHEMISTRY-II BSCCH-201
• Applications of coordination compounds,

• Name of coordination compounds,
• How to determine oxidation state of metal ion in a coordination
compound.
6.2 INTRODUCTION
You have already studied in your earlier classes that there are two types of
compounds. The compounds that can easily dissociate into their constituent ions in
aqueous medium are called simple salts and double salts such as NaCl, MgCl2,
FeSO4.(NH4)2SO4.6H2O, K2SO4.Al2(SO4)3.24H2O, etc. On the otherhand, the
compounds which donot dissociate into their constituent ions in any solvent are
known as coordination or complex compounds such as [Cu(NH3)4]2+. Transition
metals have an ability to form a number of coordination compounds due to their
small size, high charge and presence of empty d orbitals on the metal ion. A
compound formed from the union of metal ion (an electron deficient species, central
metal atom/ion; Lewis acid) and electron rich species (ligand; Lewis base) which can
donate one electron pair is called coordination compound or complex compound. The
coordination compounds can be represented by the general formula, [MLn]±m , where
M is a metal ion, L is electron rich species; n is the number of L attached to the
metal atom/ion and m is the charge on complex ion.
Some metal complexes were prepared and used in the eighteenth century in
the form of metal salts and vegetable extracts as paints. The first well known
coordination compound was Prussian blue, Fe4[Fe(CN)6]3 in the beginning of
eighteenth century. In 1798, CoCl3.6NH3 was discovered. Werner gave a theory to
understand the bonding in such compounds about a century later in 1893. We are
studying chemistry of coordination compounds because they have many applications
in analytical / environmental chemistry, metallurgy, biological systems, industries
and medicine.
6.3.2 Ligands and their types
The electron rich species, which may be charged species, e.g. Cl-, CN-, NO2-, etc or
neutral species e.g. H2O, NH3, NH2CH2C2NH2, CO, NO, etc., that can donate an
electron pair to the metal atom/ion are called ligands.
6.2.1 Types of ligands
The ligands can be classified in the following ways:
6.2.2.1 Type I- Based on electron accepter/donor properties of the ligand
UTTARAKHAND OPEN UNIVERSITY Page 97

• σ (sigma) donor ligands are those ligands which can only donate
electron pair to the meal ion, e.g. H2O, NH3, F-. These ligands are also
known as weak field ligands.
• σ (sigma) donor and Π (pi) accepter ligands are those ligands which
can donate electron pair and also have a tendency to accept electron in
their empty antibonding π molecular orbitals (MO). Such ligands can
involve in backbonding (π bond) with the metal ion. For example, CO,
CN-, NO, etc. These ligands are also known as strong field ligands.
• Π (pi) donor ligands are those ligands like benzene and ethylene,
which do not have lone pair of electrons but only π electrons for
donation to the metal atom/ion.
6.2.2.2 Type II- Based on the basis of number of donor atoms in the ligand
• Monodentate or unidentate ligands
The ligands that bound to a metal ion through a single donor atom are
called as monodentate or unidentate ligands, e.g. Cl-, H2O or NH3.
These ligands can be further divided into the following subclasses on
the basis of charge (Figure 6.1).
• Ambidentate ligands
Some ligands have two or more than two different donor atoms. These
ligands can attach through any of the donor atoms. They are given
different names depending upon nature of the donor atom linked to the
metal atom. These ligands are known as ambidentate ligands e.g. NO2
(donor atom may be either N or O), SCN- (donor atom may be either S
or N), CN- (donor atom may be either C or N), S2O32- (donor atom
may be either S or N). These are also monodentate ligands.
Figure 6.1: Classification of monodentate ligands
• Polydentate (bidentate, tridentate, tetradentate, pentadentate,

hexadentate) ligands

These ligands bind to a metalatom/ion through two, three, four, five and six donor
atoms, respectively. These ligands are also known as chelating ligands. These ligands
form ring type (chelates) compounds (Table 6.1).
Table 6.1: Polydentate ligands
Ligand type Ligand name Ligand formula/structure
Bidentate oxalate ion O-
C O-
O C
H2
C CH2
1,10-phenanthroline
H2N NH2
-
H3C C C CH3
N N
ethylenediamine (en)
O- OH
dimethylglyoxime N
OH
8-hydroxyquinoline

Tridentate diethylenetriamine (H2C)2

H
N (CH2)2
NH2
H2N
Tetradentate triethylenetetramine NH2(CH2)2NH(CH2)2NH(CH2)2NH2
porphyrin ring NH N
N HN
Pentadentate ethylenediaminetriacetato 3-
C H2
COO*- H2 C COO-*
N* (CH2)2N*
H
COO*- H2
C
Hexadentate ethylenediaminetetraacetate O O
(EDTA) -O C CH2 H2C C O-

H2 H2
N C C N
- O-
O C CH2 C
O O
Red colour letter in the structure represents the donor atom of the ligand.
6.2.2.3 Type III- Based on size of ligand
• Chelating ligands are those ligands that bind via more than one atom and
form chelate complexes (ring complexes). These complexes are more stable
than complexes formed from monodentate ligands. The enhanced stability is
known as the chelate effect.
• Macrocyclic ligands are the chelating ligands that can form a large ring and
surround the central atom or ion partially or fully and bond to it. The central
atom or ion resides at the centre of the large ring. This complex formed is more

rigid and inert as compared to the chelate compound and is known as

macrocyclic complex. Heme is a macrocyclic complex in which the central
iron atom is present at the centre of a porphyrin macrocyclic ring.
Dimethylglyoximate complex of nickel is a synthetic macrocycle formed from
the reaction of nickel ion with dimethylglyoxime (Fgure 6.2) in ammonical
medium.
H
O O
H3C CH3
N N
Ni
N N
CH3
H3C
O O
Figure 6.2: Nickel(II) bisdimethylglyoximate
The order of ligands to form stable compounds
Macrocyclic > Chelate > Monodentate
6.2.2.4 Type IV- Based on their use in reactions
• Actor ligands are those ligands which take part in chemical reaction.
• Spectator ligands are tightly coordinating polydentate liangds

which do not take part in a chemical reaction. Phosphines, allyl groups
in catalysis, trispyrazolylborates (Tp), cyclopentadienyl ligands (Cp)
and many chelating diphosphines such as 1,2-bis(diphenylphosphino)
ethane ligands (dppe) are spectator ligands.
6.2.2 Complex ion
A complex ion is an ion in which the metal ion is present at the center and a
definite number of ligands surround it. The complex ion is enclosed in a large
bracket. e.g.
[Cu(NH3)4]2+, [Ni(CN)4]2-
6.2.3 Coordination number (CN)

The coordination number of a metal atom/ion is the number of ligands attached

to it in a complex compound. Coordination number may be two, three, four,
five, six, seven, eight, nine or even higher in case of lanthanides and actinides
(Table 6.2). Coordination number depends on size, charge and electronic
configuration and nature of meal/ion and ligands. For example,
– large metal atoms show high CN

– bulky ligands reduce coordination number
– Lewis bases easily donate electrons to metals and metals with lesser number
of electrons can easily accept electrons.
Table 6.2: Coordination numbers (CN) and geometry of coordination

compounds
S.N. CN Metals Ligands Type of Geometry of complex
geometry
1 2 d-electron rich Large ligands Linear L M L
+
metals like Cu , CH3
Ag+
[Ag(NH3)2]+
[Cu(NH3)2]+
(uncommon)
2 4 Small, high Large Tetrahedral L
oxidation state,
lower d metals L M L
d8 metal atoms or Pi bonding Square L L

ions such as Ni2+, ligands planar M
Rh+, Ir+, Pt2+, Pd2+, L L
Au3+, 6+
[Cu(NH3)4]2+
[Zn(CN)4]2-
3 5 Allows fluxionality Trigonal L

L
and Berry bipyramida
pseudorotation l M L
[Ni(CN)5]3- L
L
(Rare)
Square

pyramidal M
L
L L
L
4 6 [Fe(H2O)6]2+ All types of Octahedral L
[Ni(NH3)6]2+ ligands L
4- L M L
[Fe(CN)6] L
(Very common) L
L
L
(Rare) Three Trigonal
L L
bidentate prismatic
L
ligands such L
as dithiolates
or oxalates
5 7 Generally shown by Capped L L
L L
rare earths octahedron L M
K3[NbOF6] L
L
(Very rare)
L
Capped L
trigonal
L M L
L
prism L
L
L
Uncommon L
M L
Pentagonal L
L
L
bipyramid L
6 8 Generally shown by For eight Dodecahed

rare earth metal equivalent ron
ions (Very rare) ligands
Large metal ions

(Rare) Cube

Uncommon
Square
antiprism
Common
Hexagonal
bipyramida
l
7 9 Very rare Three-face

centred M
trigonal
prism
(Tricapped
trigonal
prism)
8 10 Generally shown by Bicapped
rare earth elements square
antiprism M
AA 11 Very rare All-faced

capped M
trigonal
prism
(Octadecah
edron)
10 12 Generally shown by Icosahedro
rare earths n
M
6.2.4 Coordination sphere
The coordination sphere of a coordination compound comprises the central metal

atom/ion and ligands attached to it. The coordination sphere is enclosed in brackets [
].

Coordination sphere [Co(NH3)6]3+
6.2.5 Counter ions:
The ions excluding the coordination sphere are called as counter ions. In K4[Fe(CN)6]
compound, K+ is the counter ion.
6.3 WERNER’S COORDINATION THEORY AND ITS
EXPERIMENTAL VERIFICATION
Alfred Werner in 1893 suggested a new theory for explaining the nature of
bonding in coordination compounds known as Werner’s theory. According to this
theory, there are two kinds of valences of metal atom/ion in coordination compounds:
primary and secondary valences
• The attachment of species that satisfy both primary as well as

secondary valences is shown by solid-broken line
Explanation of structure of Co(III) complexes on the basis of Werner’s
theory (Figure 6.3).
NH3 Cl- Cl-

Cl-
H3N NH3 H3N NH3 H3 N NH3

-Cl Co 3+ -
Cl Co3+ Cl- Co3+ Cl-
H3N NH3 H 3N NH3 H3 N NH3
-
Cl NH3 NH3 Cl-
Cl-
H 3N NH3
Co3+
Cl- NH3
Cl-
(a) (b) (c) (d)
Figure 6.3 Structure of Co(III) amine complexes on the basis of Werner’s

theory. In all the complexes the primary valency of Co is 3 and secondary
valency is 6. (a) CoCl3.6NH3 [Co(NH3)6]Cl3 (b) CoCl3.5NH3
[Co(NH3)5Cl]Cl2 (c) CoCl3.4NH3 [Co(NH3)4Cl2]Cl (d) CoCl3.3NH3
[Co(NH3)3Cl3]
Werner’s theory can be explained on the basis of experimental evidences

• Molar conductivity measurement method
Compounds [Co(NH3)6]Cl3, [Co(NH3)5Cl]Cl2, [Co(NH3)4Cl2]Cl and

[Co(NH3)3Cl3] show decreasing order of conductivity due to the formation
of 4 ions, 3 ions, 2 ions and no ions, respectively in solution.
[Co(NH3)6]Cl3 [Co(NH3)6]3+ + 3Cl- (1 complex ion + 3 chloride

ions)
[Co(NH3)5Cl]Cl2 [Co(NH3)5Cl]2+ +2Cl- (1complex ion +2chloride
ions)
[Co(NH3)4Cl2]Cl [Co(NH3)4Cl2] + +Cl- (1 complex ion+1chloride
ion)
[Co(NH3)3Cl3] [Co(NH3)3Cl3] (No ions)
• Precipitation method
When cobalt ammine chloride complexes react with AgNO3, some of the
Cl- ions get precipitated with AgNO3 to form AgCl. The number of Cl-
ions that are ionisable and present outside the coordination sphere can
react with AgNO3. Thus, coordination compounds [Co(NH3)6]Cl3,
[Co(NH3)5Cl]Cl2 and [Co(NH3)4Cl2]Cl react with 3, 2 and 1 mole of
AgNO3 to form 3, 2 and 1 mole of AgCl, respectively as there are 3, 2 and
1 ionisable Cl- ions, respectively.
Defects of Werner’s theory
• Although the theory describes the structure of many compounds, it can’t
explain the nature of bonding between metal atom/ion and ligands.
• Werner’s theory was unable to explain why 4- and 6-coordination numbers
are the preferred coordination numbers.
6.4 EFFECTIVE ATOMIC NUMBER CONCEPT (EAN
CONCEPT)
This rule is given by English Chemist Nevil V. Sidgwick. Effective atomic

number (EAN) is the total number of electrons in metal atom/ion (atomic number)
plus the electrons gained from ligands. This EAN is the atomic number of a noble
gas. Therefore, EAN decides stability of coordination compound. If a coordination
compound follow EAN rule, than it is stable one.
EAN= Atomic number of metal atom/ion + number of e- donated by ligands or 2 x
number of ligands (as each ligand can donate two electrons to metal atom/ion).
For [Co(NH3)6]3+
Atomic number of Co=27; Atomic number of Co3+=24; there are six ligands hence
electrons donated by 6 ligands = 6 x 2

EAN = 24 + (6 x 2) = 36 (atomic number of Krypton; Kr)
For [Ni(CO)4]
Atomic number of Ni=28; there are four ligands hence electrons donated by 4 ligands
=4x2
EAN = 28 + (4 x 2) = 36 (atomic number of Krypton; Kr)
For [Fe(CN)6]4-
Atomic number of Fe=26; Atomic number of Fe2+=24; there are six ligands hence
electrons donated 6 ligands = 6 x 2
EAN = 24 + (6 x 2) = 36 (atomic number of Krypton;
For [Ag(NH3)4]+
Atomic number of Ag=47; Atomic number of Ag+=46; there are four ligands, hence
electrons donated by 4 ligands = 4 x 2
EAN = 46 + (4 x 2) = 54 ( atomic number of Xenon; Xe)
= 54 (atomic number of Xenon; Xe)
For [V(CO)6]-
Atomic number of V=23; Atomic number of V- = 24; there are six ligands hence
electrons donated 6 ligands = 6 x 2
EAN = 24 + (6 x 2)
= 36 (atomic number of Krypton; Kr)
For [Mn(CN)4]2-
Atomic number of Mn=25; Atomic number of Mn2+ = 23; there are four ligands
hence electrons donated 4 ligands = 4 x 2
EAN = 23 + (4 x 2)
= 31
Not obeying EAN rule as 31 is not the atomic number of any noble gas
6.5 CHELATE
A chelate is a stable coordination compound in which a metal atom/ion

attached to lingand(s) with more than one donor site producing one or more ring
(Figure 6.4). These compounds are also called as cyclic or chelated complexes and

the formation of such rings is called as chelation or cyclisation. Classification of

chelates depends upon the type and nature of chelating ligands such as bidentate
chelates (with bidentate ligands), tridentate chelates (with tridentate ligands) etc
N H2 O
N
II
Fe N
N O
O
N
HO
Fe
O N
OH O H2
O
Figure 6.4: Chelates of iron
6.5.1 Applications of chelates:
Chelates are useful in water softening, medicical and analytical chemistry and
different industries such as chemical and food industry and agriculture.
In water softening:
Calcium (Ca2+) and magnesium (Mg2+) ions are responsible for hardening of
water. These ions on reaction with soaps precipitate out. In the presence of
chelating ligands such as polyphosphates and polydentate amino acids, no
precipitation occurs as these ions form chelate complexes with polyphosphate
and polydentate ligands present in soap and thus, soften water.
In food industry:
Metal-amino acid chelates are helpful in enhancing mineral absorption such as

Ferrous bis-glycinate. Chelates are also used to preserve fruits, fruit juices, food
stuffs etc.
In agriculture:
Metal chelates are used as common components of fertilizers in agriculture.

Micronutrients such as manganese, iron, zinc and copper are required for the
overall health of the plants. These micronutrients along with EDTA form
chelate fertilizers. Presence of chelates enhance uptake of micronutrients by the
plants. Chelating ligands are also used to detoxify poisonous metal such
as mercury, arsenic and lead present in polluted water.
In medical field:

Tetracycline and quinolone can form chelate with Fe2+, Ca2+ and Mg2+ ions and
thus, these chelates can be used as suppliments of these ions. As EDTA softens
the dentin, it is used in root canal treatment as an intracanal irritant. Chelates
of gadolinium are used as contrast agents in MRI scans. Metal poisoning can be
decreased by chelation with EDTA as toxic metals such as mercury, arsenic,
lead and other radioactive metals can be excreted without further interaction
with the body by converting them into chemically inert form (EDTA metal
complex). Chelation is also used in the treatment for autism.
Chemical applications:
Homogeneous catalyst such as ruthenium(II) chloride chelated with BINAP (a
bidentate phosphine) is used in Noyori asymmetric hydrogenation and
asymmetric isomerisation for the manufacture of synthetic (–)-menthol. Bio-
Rust and Evapo-Rust are chelating agents used for the removal of rust from
iron and steel. Metal chelates are also used in dyeing industry.
Physiological chemistry (in human body):
In body fluids, citric, malic and tartaric acids, the natural chelating agents,
keep the metal ions away from precipitation. The other physiologically
important chelates are haemoglobin, vitamin B12, chrorophyll, cytochrome and
plastocyanine
6.5.2 Stability of chelates:
Stability of chelates can be explained on the basis of standard free energy

change (∆Gº), standard entropy change (∆Sº), steric effect, size of ring and
number of rings.
• Change in thermodynamic variables:
Consider formation of coordination compounds of cadmium with

monodentate and bidentate ligands, e.g.,
[Cd(H2O)4]2+ + 4CH3NH2 [Cd(CH3NH2)4]2+ + 4H2O.....i, ∆Hº=-

ve
Monodentate ligand Non-chelated compound
1 molecule 4 molecules 1 molecule 4 molecules
[Cd(H2O)4]2+ + 2en [Cd(en)2]2+ + 4H2O...............ii, ∆Hº=-ve

1 molecule 2 molecules 1 molecule 4 molecule
Bidentate ligand Chelated compound

Stability of the product in a chemical reaction can be explained by the

following reaction:
∆Gº = (∆Hº) - (T∆Sº) where ∆Hº is change in enthalpy and T is temperature

You already know that a product will be stable if value of ∆Gº is negative (-ve)
and value of ∆Sº is positive (+ve).
During the formation of a nonchelated compound with monodentate ligands (i), the
number of molecules on both side is equal (5 molecules each side), hence ∆Sº will be
zero and the value of ∆Gº will be negative.
With bidentate ligand (ii), number of product molecules (5 molecules) is greater than
the number of reactant molecules (3 molecules). Hence, entropy of product is greater
than reactants and thus, ∆Sº will be positive and value of ∆Gº will become more
negative. Thus, it may be concluded that ∆Gº has more negative value for chelates
which are formed by the reaction of bidentate or other polydentate ligands and
therefore, are more stable as compared to the nonchelated compounds.
• Steric effect:
When a group is present on the donor atom or near the donor atom of a ligand,
the metal ligand bonds becomes week and thus, lower the stability of that
compound. This effect is known as steric effect or steric hinderence. Among
complex compounds I and II, compound II will be more stable as in
compound I, there is one methyl group near the donor atom N which causes
steric effect.
Donor atom Group causing steric effect
Ni Ni
N N
CH3
O- 2 O-
and 2
(I) (II)
• Size of chelate ring:

Chelate ring having 5-members give stable compounds and chelate rings
having more or less than 5-members are generally less stable. This happens
because the stability constant for 5-membered chelate is the highest.
• Number of rings:

Greater the number of rings, more will be the stability of chelate. As the
number of donor atoms in a chelating ligand increases, stability increase due
to the increase in the number of chelate rings.
Stability order of the chelates with the bi- and tridentate ligands.
en (number of rings=1) < trien (number of rings = 3)
6.6 NOMENCLATURE OF COORDINATION

COMPOUNDS
The coordination compounds are named according to the following steps suggested
by IUPAC (International Union of Pure and Applied Chemistry) (Figure 6.5).
Step I: Naming of ions
• The positively charged metal ion is written first followed by the
negative ions in ionic compounds.
- FeCl2 : Iron (II) chloride
- KCl : Potassium (I) chloride
• Name of non-ionic or molecular complexes are written as one word
without any gap.
- K4[Fe(CN)6]: Potassium (positive ion) hexacyano(ligand)ferrate
(central ion)(III) (non ionic compound)
- [Pt(NH3)4]4+: Tetraammineplatinum (IV)
The name in red is one word without any gap as it is a complex
ion.
Step II: Naming coordination sphere
In case of the complex ion (coordination sphere), name of ligands written first
than only central metal atom/ion along with its oxidation number in
parenthesis should be written.
[Ni(CO)4] - Tetracarbonylnickel(0)
[Pt(NH3)4]4+ - Tetraammineplatinum(IV)
Ending of name: If the complex is anionic, the suffix ate is added to the
name of metal along with oxidation state in parenthesis.
K2[HgCl4] Potassium tetrachloridomercurate(II)
Step III: At the last, the name of anion which is present on the outside of the
coordination sphere should be written, e.g.,
[Pt(NH3)4]Cl4 - Tetraammineplatinum(IV) chloride

[Co(NH3)6]Cl3 - Hexaamminecobalt(III) chloride
Names of some common ligands

Ligands are named alphabetically along with a prefix (di, tri, tetra, penta etc
for simple ligands and bis, tris, tetrakis etc. for complicated ligands) of their
number. Some of the common ligands are given in Table 6.3.
Table 6.3: Name and abbreviation of ligands

Type of ligands Category of Formula Name
ligand
H2 O Aqua
Neutral (written NH3 Ammine
as a neutral
Simple ligands molecule)
CO Carbonyl
NO Nitrosyl
F- fluorido
Anionic (-o at Cl- chlorido
the end of
ligand’s name)
Br- bromido
I- iodido
OH- hydroxido
CN- cyanido
C2O42- oxalato
NO3- nitrato
CO32- carbonato
NH2- amido
H- hydrido
NO2- nitrito-N
ONO- nitrito-O
O2- oxido
O22- peroxido
N3 - azido
S2- sulphido
C 6 H5 - phenyl
NH2- amido
NH2- imido
C 5 H5 - cyclopentadienyl
Cationic (-ium NO+ Nitrosonium
at the end of NO2+ nitronium
ligand’s name)
en C2H8N2 ethylendiamine
py C 5 H5 N pyridine
ox C2O42- oxalato
dmso (CH3)2SO dimethyl sulfoxide

EDTA C8H12N2O84- ethylenediamine

Complicated tetraacetato
ligands with gly NH2CH2CO2- glycinato
abbreviation oxine C9H7NO 8-hydroxyquinolinato
phen C12H8N2 1,10-phenanthroline
dmg C4H7N2O2 dimethylglyoximato
ur NH2CONH2 urea
tu H2NCSNH2 Thiourea
Naming of bridging complexes
The bridging groups in the complexes are written first with a prefix µ by separating it
from the complex by hyphen (-). This µ is used every time for each bridging ligand.
Thereafter, the other liands with number prefix (di, tri or bis-, tris- etc.), metal with
oxidation state in bracket and at last the negative species outside the cordination
sphere.
Type of complex/ Formula of Name of compound
compound compound
[Ni(CO)4] Tetracarbonylnickel(0)
[Fe(C5H5)2] Bis(cyclopentadienyl)iron (II)
Complexes [Pt(en)2Cl2] Dichlorobis(ethylenediamine)platinum(
with neutral II)
coordination [Hg(CH )
3 2 ] Dimethylmercury(II)
sphere [Mn3(CO)12] Dodecacarbonyltrimanganese (0)
[Ni(PF3)4] Tetrakis(trifluorophosphine)nickel(0)
K2[HgCl4] Potassium tetrachloromercurate(II)

K4[Cu(CN)6] Potassium hexacyanocuperate(II)
Na3[Co(NO2)6] Sodium hexanitrito-Ncobaltate(III)

K2[Fe(CN)5NO] Potassium pentacyanonitosylferrate(III)
Na[Au(CN)2] Sodium dicyanoaurate(I)
K3[Cr(CN)6] Potassium hexacyanochromate(III)
Simple Complexes K4[Ni(CN)4] Potassium tetracyanonickelate(0)
(one ion with anionic Na3[Fe(C2O4)3] Sodium trioxalatoferrate(III)
is coordination Fe[Fe(CN)6] Iron hexacyanoferrate(III)
complex sphere K4[Fe(CN)6] Potassium hexacyanoferrate(II)
and K3[Fe(CN)6] Potassium hexacyanoferrate(III)
other Na2[ZnCl4] Sodium tetrachlorozincate(II)
simple [Cr(C2O4)3]3- Trioxalatochromate(III)
ion)
[Co(N3)(NH3)5)]SO4 Pentaammineazidocobalt(III) sulphate
[Cr(H2O)6]Cl3 Hexaaquachromium(III) chloride
[Cr(NH3)6]3+ Hexaammine chromium(III) ion

[Cr(H2O)4Cl2]NO3 Tetraaquadichlorochromium(III)
nitrate
Complexes
with cationic
coordination
sphere
[Cu(NH3)2(en)]Br2 Diammine(ethylenediamine)copper(II)
bromide
[PtClBr(NH3)py] Amminebromochloropyridinepalatinu
m(II)
[CuCl2(CH3NH2)2] Dichlorobis(methylamine)copper(II)
Complex ligand ion
[Co(NH3)5ONO]SO4 Pentaamminenitritocobalt(III) sulphate
[Co(NH3)2(H2O)2(CN Diamminediaquadicyanocobalt(III)
)2]Cl chloride
[Cr(NH3)2(H2O)3(OH Diamminetriaquahydroxochromium(III
)](NO3)2 ) nitrate
[Pt(NH3)4][PtCl4] Tetraammineplatinum(II)
tetrachloropalatinate(II)
(Firstly complex
Both cation and anion cation is named then
are complex only the complex
anion)
[Cr(NH3)5(NCS)][Zn Pentaammineisothiocyanatochromium
Cl4] (III) tetrachlorozincate(II)
Octaaqua-µ-dihydroxo-diiron(III)
sulphae
or
Tetraammineiron(III)-µ-dihydroxo-
OH
Fe NH3
NH3 Fe SO4
tetraammineiron(III)
4 4 2
OH
or
????? µ-dihydroxo-octaaquadiiron(III)
sulphate
or
Bridging complex µ-Hydroxo-tetraaquairon(III)µ-
???? hydroxo-tetraaquairon(III) sulphate
OH
µ-Dihydroxo-octaamminedicobalt(III)
NH3
4
Co
Co NH3
4
NO3
4
nitrate
OH

NH2
µ-Amido-µ-hydroxo-
en
2
Co Co en
2
Cl3 tetrakis(ethylenediammine)dicobalt(III)
OH
sulphate
Calculation of oxidation number of metal atom/ion
In coordination compound, [M(L)n]± or [M(L)n], oxidation state of M can be

calculated as Oxidation number of M + Oxidation number of L x n = ± or 0
For example,
1) Oxidation number (ON) of Co in [Co(NH3)6]3+
ON of Co + 0 (ON of NH3 is 0) = +3
Hence, ON of Co will be +3.
2) Oxidation number (ON) of Ni in [Ni(CO)4]

ON of Ni + 0 (ON of CO is 0) = 0
Hence, ON of Ni will be 0.
6.7 SUMMARY
In this Unit, you have studied the fillowing:

• Simple or double salts dissociate into their constituent ions while
coordination compounds break up into complex ion and its counter
charged ion.
• Coordination compounds comprise of two main parts; central metal
atom/ion and ligands coordinated to the metal atom/ion.
Coordination number or secondary valency

Coordination sphere
Ionisable part
Co(NH3)6 Cl3
Primary valency
Central metal atom/ion Ligands
• A wide variety of ligands including monodentae, polydentate, sigma

donor, chelating, macrocyclic, actor and spectator ligands are involved
in the formation of coordination compounds with metal atom/ion.
• Monodentate lingads can donate a pair of electrons to a central metal
atom while a polydentate ligand can donate more than one pair of
electrons to the metal atom/iom. Bidentate or polydentate ligands form
cyclic compounds called chelats.
• Coordination compounds have various applications in different
industries.

• Werner suggested that coordination number and oxidation number of

metal depends on the nature of the metal.
• EAN rule decides the stability of a coordination compound.
• Nomenclature of coordination compounds is systematic.
6.8 DEFINITIONS
Chelate: Chelate is a compound containing a ligand (typically organic) bonded

to a central metal atom at two or more points.
Complex ion: A complex ion is an ion formed by the coordination of central
metal ion with one or more ligands.
Coordination compound: A compound in which a central atom/ion is bonded
to a definite number of ligands.
Coordination number: It is the number of ligands surrounding the central metal
ion.
Effective Atomic Number: It is the sum of the electrons of metal atom/ion and
electrons donated by ligand.
Ligand- Ligand is a charged or neutral molecule which can done electron pair to
the metal atom to form a coordination compound.
Primary valency: Primary valency in a coordination compound is the number of
negative ions that satisfy the positive charge on the central metal ion.
Secondary valency: Secondary valency in a coordination compound is defined
as the number of ligands that are coordinated to the central metal ion. It is equal
to the coordination number.
6.9 QUESTIONS
6.9.1 Short answer questions
1. Which of the following ligands are weak or strong?

CN-, CO, H2O, NH3
2. Classify the following ligands as pi acceptor or sigma donor ligands.
NO, Cl-, NH3, benzene
3. Give two examples of each neutral and positive monodentate ligands.
4. What is chelate effect?
5. Select monodentate and polydentate ligands among the following:
NCS-, NH3, C2O42-, EDTA, en, Cl-
6. Draw the structures of the following ligands:
EDTA, dmg, porphyrin, 8-quinoline, diethylenetriamine
7. Explain spectator ligand.
8. How chelates are useful?

9. What do you mean by complex ion?

10. Calculate oxidation state and coordination number of Fe in [Fe(CN)6]4- and
[Fe(C2O4)3]3-?
11. Draw the geometry that describe five and six‐coordinate compounds.
Which structures are more common?
12. Give the geometry of the following compounds:
(i) [Co(NH3)6]3+
(ii) [Ag(NH3)2]+
(iii) [Fe(CN)6]4-
(iv) [Fe(C2O4)3]3-
13. Define coordination sphere.
14. What is primary valency?
15. What do you mean by secondary valency?
16. On the basis of Werner’s theory, draw the structure of CoCl3.6NH3 and
CoCl3.5NH3.
17. Name the experimental methods to explain Werner’s theory?
18. Define EAN rule.
19. Who gave the concept of EAN rule?
20. Calculate EAN for [Ni(CO)4].
21. Calculate the oxidation number of the metal atom/ ion in the following
coordination complexes:
a) [CoCl4]2‐
b) [Fe(CN)6]3-
c) [Ni(CO)4]
d) [Cr(CO)6]
22. Name the following ligands:
Cl-, H-, NO+, H2O, NO3-, ONO-, NH2-, C5H5-
23. Write down the abbreviation and formula of the following ligands:
ethylenedimmine, dimethyl sulphoxide, glycine, urea, 1,10-
phenanthroline, ethylenediammine tetraacetic acid.
24. Name the following coordination compounds:
a. Na3[Co(NO2)6]
b. K4[Ni(CN)4]
c. [Mn3(CO)12]
d. K3[Fe(CN)6]
e. [Pt(NH3)4][PtCl4]
f.
OH
Co NH3
NH3 Co NO3
4 4 4
OH
25. Write down the formula of hexamminecobalt (III) chloride and

tetracarbonyl nickel (0).
26. Draw the structure of the following complexes:
a) Sodium hexafluoroslilicate (IV)

b) Ammonium diamminetetra(isothiocyanato)chromate(III)
27. How does the conductivity of [Co(NH3)6]Cl3 and [Co(NH3)5Cl]Cl2 differ
from each other?
28. Write the formula of the following compounds:
a) Octaammine-µ-amido-µ-nitridocobalt(III) nitrate
b) Dichlorobis(ethylenediamine)cobalt(III) chloride
c) Hexaaquairon(II) sulphate
d) Tetrachloroplatinate(II)
29. Which of the following complexes follow EAN rule? Give electron count in
each case.
a) [Cr(NH3)6]3+
b) [Ni(NH3)6]2+
c) [Cr(CO)6]
d)
[Mn(CN)6]4-
e) [Fe(H2O)6]2+
f) [Fe(CN)6]4+
g) [Fe(CN)6]3-
30. Explain the meaning of the terms monodentate, bidentate and tetradentate.
31. Arrange the following complexes in the increasing order of molar
conductivity:
a) [Co(NH3)6]Cl3 b) [Co(NH3)5Cl]Cl2 c) [Co(NH3)3Cl3]
6.9.2 True (T) or False (F)
a) CO and NO are pi acceptor ligands.

b) Monodentate ligands are of three types: neutral, negative and positive
c) Ethylenediammine is a monodentate ligand.
d) Monodentate ligands form more stable compounds as compared to
polydentatte ligands.
e) Spectator ligands are those ligands that take part in a chemical
reaction.
f) Coordination number is also known as secondary valency.
g) Coordination number does not decide the geometry of coordination
compound.
h) Octahedral and trigonal prismatic geometries are found in 6
coordinated compounds.
i) Chelates are the cyclic compounds.
j) More positive the value of ∆Gº, more stable the chelate.
k) Primary valency is the oxidation number or the group valency of the
metal atom/ion.
l) The attachment of species that satisfy both primary as well as
secondary valences is shown by ----------- broken line.
m) Complex [Co (NH3)3Cl3] will ionize to give two ions.

n) Complex [Co (NH3)5Cl]Cl2 will react with three molecules of AgNO3.

o) EAN stands for Effective Atomic Nuclei.
p) Abbreviation for glycine is gly.
q) IUPAC name of [Cr (NH3)6]3+ complex is hexaamminechromate (III).
r) Formula of the compound Tetrachloropalatinate (II) is [PtCl4]2-.
6.10 LONG ANSWER QUESTIONS
a) What is the difference between chelating and macrocyclic ligands?
Explain with the help of examples.
b) What are chelates? How are they important? Give examples.
c) Define ligand. Give detailed account on the classification of ligands.
d) How are the coordination compounds named? Describe with the help
of examples?
e) What is EAN rule? Give details with examples.
f) Explain Werner’s theory.
g) What do you mean by coordination number? How is it useful in
deciding the geometry of coordination compounds? Explain with the
help of examples.
h) What are primary and secondary valencies? Explain by giving suitable
examples.
i) Discuss the basic postulates of Werner’s theory of coordination
compounds. Explain structure of CoCl3.6NH3, CoCl3.5NH3,
CoCl3.4NH3, CoCl3.3NH3 on the basis of Werner’s theory.
j) Explain in detail about the stability of chelates.
6.11 ANSWERS TO SHORT ANSWER QUESTIONS
Short answer questions:

1. Weak ligands- H2O, NH3; strong ligands - CN-, CO
2. pi acceptor ligands- NO, C6H6; sigma donor ligands- Cl-, NH3
3. Neutral monodentate ligands- NH3, H2O; positive monodentate ligands-
NO+, NH2NH3+.
4. Chelating ligands are those ligands that bind via more than one atom and
form chelate complexes (ring complexes). These complexes are more stable
than complexes formed from monodentate ligands. The enhanced stability is
known as the chelate effect.
5. Monodentate ligands: NCS-, NH3, Cl-; Polydentate ligands- C2O42-, EDTA,
en.
6. EDTA:

O O
-
O C CH2 H2C C O-
H2 H2
N C C N
C O-
O- C CH2
O O
dmg:
-
H3C C C CH3
N N
O- OH
porphyrin:
NH N
N HN
8-hydroxyquinoline:
diethylene triamine:
H
(H2C)2 N (CH2)2
NH2
H2N
7. Spectator ligand, a tightly coordinating polydentate liangd that do not take
part in a chemical reaction.
8. Chelates are useful in water softening, medical and analytical chemistry
and different industries such as agriculture, chemical and food.
9. A complex ion is an ion in which the metal ion presents at the centre and a
definite number of ligands surround it. The complex ion is enclosed in a
large bracket.
10. Oxidation state and coordination number of Fe in [Fe (CN)6]4- is +2 and
six while in [Fe(C2O4)3]3- is +3 and six, respectively.
11. Geometry that describes five coordinated compounds:
Trigonal bipyramidal and Square pyramidal; Trigonal bipyramidal is the
most stable geometry.
Geometry that describes six coordinated compounds.

Octahedral and Trigonal prismatic; Octahedral is the most stable

geometry.
12.
(i) Octahedral
(ii) Linear
(iii) Octahedral
(iv) Octahedral
13. The coordination sphere of a coordination compound comprises the

central metal atom/ion and ligands attached to it. The coordination sphere
is enclosed in brackets [ ].
14. Primary valency is the oxidation number or the group valency of the metal
atom/ion, for example +2 for copper (Cu).
15. Secondary valency is the coordination number or the number of ligands
coordinated to the metal aom/ion such as 6 for cobalt (Co).
16. CoCl3.6NH3 CoCl3.5NH3.
NH3
Cl-
H3N NH 3
-Cl Co3+
H3N NH3
-
Cl NH 3
Cl-
H3N NH3
-
Cl Co3+ Cl-
H 3N NH3
NH3
17. Molar conductivity measurement method and precipitation method.

18. Effective atomic number (EAN) is the total number of electrons in metal
atom/ion (atomic number) plus the electrons gained from ligands. This
EAN is the atomic number of a noble gas. Therefore, EAN decides
stability of coordination compound. If a coordination compound follows
EAN rule, than it is stable one.
19. English Chemist Nevil V. Sidgwick gave the concept of EAN rule.
20. EAN for [Ni(CO)4]
Atomic number of Ni=28; there are four ligands hence electrons donated by 4
ligands = 4 x 2 = 8
EAN = 28 + 8
= 36 (atomic number of Krypton; Kr)
21. Oxidation number of the metal in the following coordination complexes:

a) +2
b) +3
c) 0
d) 0
22. Cl- = chloro; H- = hydrido; NO+ = nitrosonium; H2O = aqua; NO3- =
nitrato; ONO- = nitrito-N; NH2- = amido; C5H5- = cyclopentadienyl.
23. ethylenedimmine = en; dimethyl sulphoxide = dmso; glycine = gly; urea =
ur; 1,10-phenanthroline = phen; ethylenediammine tetraacetic acid =
EDTA.
24. a. Na3[Co(NO2)6] = Sodium hexanitrito-N cobaltate (III)
b. K4[Ni(CN)4] = Potassium tetracyanonickelate (0)
c. [Mn3(CO)12] = Dodecacarbonyltrimanganese (0)
d. K3[Fe(CN)6] = Potassium hexacyanoferrate (III)
e. [Pt(NH3)4][PtCl4] = Tetraammineplatinum(II) tetrachloropalatinate
OH
Co NH3
NH3 Co NO3
4 4 4
OH
f. = µ-Dihydroxo-octaamminedicobalt(III) nitrate
25. Hexamminecobalt(III) chloride = [Co(NH3)6]Cl3

tetracarbonylnickel(0) = [Ni(CO)4]
26. a) Na2[SiF6]
b) NH4[Cr(NH3)2(NCS)4]CrO4
27. Compounds [Co(NH3)6]Cl3 and [Co(NH3)5Cl]Cl2 show decreasing order of
conductivity due to the formation of 4 ions and 3 ions, respectively in
solution.
28.
a.
NH2
Co NH3
NH3 Co NO3
4 4 4
NO2
b. [Co(en)2Cl2]Cl
c. [Fe(H2O)6]SO4
d. [PtCl4]2-
29.
a. EAN = 37 (not follow EAN rule)
b. EAN = 38 (not follow EAN rule)
c. EAN = 36 (follow EAN rule)
d. EAN = 35 (not follow EAN rule)
e. EAN = 36 (follow EAN rule)
f. EAN = 36 (follow EAN rule)
g. EAN = 35 (not follow EAN rule)
30. The ligands that bound to a metal ion through a single donor atom; two
donor atoms and four donor atoms are called as monodentate, bidentate
and tetradentate ligands, respectively.

31. c) [Co(NH3)3Cl3] b) [Co(NH3)5Cl]Cl2 a) [Co(NH3)6]Cl3
True (T) or False (F)
a. T
b. T
c. F
d. F
e. F
f. T
g. F
h. T
i. T
j. F
k. T
l. F
m. F
n. F
o. F
p. T
q. F
r. T
6.12 BIBLIOGRAPHY
a) B. R. Puri, L. R. Sharma and K. C Kalia, Principles of Inorganic

Chemistry, Milestone Publishers & Distributors, Meerut, 2013.
b) file://localhost/H:/coordination%20bcoordination%20cheemistry/Coord
ination %20 Chemistry.htm
c) G. S. Sodhi, Textbook of Inorganic Chemistry, Viva Books Private
Limited, New Delhi, 2013.
d) http://www.rsc.org/pdf/tct/df-chapter.pdf
e) http://www.trentu.ca/chemistry/chem321h/lectures/lecture321coordinat
ion_compounds.pdf
f) https://www.wou.edu/las/physci/ch412/ligand.htm
g) M. S. Yadav. Quick Review in Inorganic Chemistry, Anmol
Publications Pvt. Ltd., New Delhi, 2004.
h) P. Mishra Advanced Inorganic Chemistry, Jagdamba Publishing
Compan, New Delhi, 2011.
i) R. Gopalan, and V. Ramalingam. Concise Coordination Chemistry,
Vikas Publishing House Pvt Ltd., New Delhi, 2005.
j) R. L. Madan, Chemistry for degree students, B.Sc. II year, S. Chand &
Company Ltd., New Delhi, 2011.
k) S. Prakash, G. D. Tuli, S. K. Basu, and R. D. Madan. Advanced
Inorganic Chemistry, Vol. II, S. Chand & Company Ltd., New Delhi,
2007.

l) W. U. Malik, G. D. Tuli, and R. D. Madan. Selected Topics in

Inorganic Chemistry, S. Chand Publication, Delhi, 2013.

UNIT 7-ISOMERISM OF COORDINATION

COMPOUNDS
CONTENTS:
7.1 Objectives
7.2 Introduction
7.3 Isomerism in coordination compounds
7.3.1 Structural isomerism
7.3.1.1 Linkage isomerism
7.3.1.2 Ionization isomerism
7.3.1.3 Coordination isomerism
7.3.1.4 Hydrate isomerism
7.3.1.5 Ligand isomerism
7.3.1.6 Coordination position isomerism
7.3.2 Methods for identification of structural isomers
7.3.3 Stereoisomerism
7.3.3.1 Geometrical isomerism
7.3.3.1.1 Four-coordinated compounds
7.3.3.1.2 Six-coordinated compounds
7.3.3.2 Optical isomerism
7.3.3.2.1 Four-coordinated compounds
7.3.3.2.2 Six-coordinated compounds
7.3.3.3 Importance of stereoisomerism
7.3.3.4 Methods for identification of stereoisomers
7.4 Valence bond theory of transition metal complex
7.5 Summary
7.6 Definitions
7.7 Questions for practice
7.6.1 True or false
7.6.2 Fill in the blanks
7.6.3 Short answer questions
7.8 Bibliography
7.9 Long answer questions
7.10 Answers
7.1 OBJECTIVES
At the end of this unit, you will be able to describe with the following:
• Isomerism
• Isomer
• Cis isomer
• Trans isomer

• Enantiomer
• Chirality
• Structural isomers
• Laevorotatory (l)
• Dextrorotatory (d)
• Geometrical isomerism
• Optical isomerism
And also gain the knowledge of:
• The types of isomerism possible in coordination compounds
• Importance of isomerism
7.2 INTRODUCTION
The objectives of this unit are to familiarize you with the isomerism in
coordination compounds and its types. The coordination compounds which have the
same chemical formula but different ways of attachment of ligands are called as
isomers. These isomers have different physical and chemical properties. The
phenomenon that gives rise to the isomers is known as isomerism. There are two
main types of isomerism in coordination compounds; structural and stereo-
isomerism. Our focus will be on both structural isomerism and stereoisomerism.
Structural isomerism is due to the different bond arrangement of atoms in
coordination compound around the central metal atom/ ion while stereoisomerism
arises due to different three-dimensional arrangement of atoms in space.
7.3 Isomerism in coordination compounds
The coordination compounds having the same molecular formula but different
arrangement of atoms/ groups around the central metal/ ion are called isomers and the
phenomenon which gives rise to isomers is called as isomerism. The isomers have
different physical and chemical properties. There are two main types of isomerism:
(1) Structural isomerism and; (2) Stereoisomerism. These can be further sub-divided
into several types. Various types of isomerism exhibited by coordination compounds
are shown in Figure 7.1.

Isomers (Compounds with same formula

but different atom arrangement)
Structural Isomers Compounds with Stereoisomers Compounds with same

different connections between atoms connectivity but different spatial
arrangement
Linkage isomers Same ligand Diastereomers (Geometric isomers)
connectedby different atoms Non-mirror images OR different
coordination polyhedra
Ionization isomers Give
ifferent ions in solution
Optical isomers (Enantiomers)
Mirror image isomers
Polymerization isomers Same
empirical formula but different
molar mass
Coordination isomers Different

ligand sets in complex cation
and anion
Hydration isomers Contain
different numbers of waters
inner/outer sphere
Figure 7.1: Types of isomerism in coordination compounds
7.3.1. Structural Isomerism
Structural isomerism is also known as constitutional isomerism. The

molecules have same number of atoms which differ in their structure or bonding. The
different chemical formulae of structural isomers are either due to difference in
ligands that are bonded to the central atoms or the mode of bonding of individual
ligand (which atom of the ligand is bonded to the central atom). Now we will discuss
the various types of structural isomerisms one by one in brief.
7.3.1.1 Linkage Isomerism

Complexes having ambidentate ligands like SCN- / NCS-, CN-/NC- and NO2-
/ ONO- (capable of coordinating in more than one way) show linkage isomerism.
The two isomers differ from each other by the linkage atom attachment to the central
atom/ ion. The ligand can have more than two donor atoms but should be joined to
the central atom/ ion via only one atom (unidentate ligand). The formula of the
compound is same but their properties are entirely different. The name of the ligands
is also changed according to their donor atom. When donor atom is N, NO2- is called
as nitro, while it is called nitrito if the donor atom is O atom (Figure 7.2).

2+ 2+
NH3 NH3
H 3N NH3
H 3N NH3
Co
Co H 3N NH3
H 3N NH3
O
N N
O O
O
Pentaamminenitrocobalt(III) Pentaamminenitritocobalt(III)
Nitro isomer (N is the donor atom) Nitrito isomer (O is the donor
atom)
Figure 7.2: Linkage isomerism
7.3.1.2 Ionization Isomerism
This is the phenomenon by which ligands present inside the coordination

sphere and anion or neutral molecule present outside the coordination sphere can
exchange their place. The central metal / ion and the other ligands except one that is
exchangeable are similar in both the isomers. Thus, ionization isomerism is the
exchange of ions between coordination sphere and ionization sphere. The physical
and chemical properties of the two isomers are entirely different as they give different
ions on dissolving in suitable solvent. Two octahedral ionization isomers will have
five identical ligands while the sixth ligand will be different. In case of tetra
coordinated isomers, three ligands will be identical and the fourth one will be the
different one. The different ligand in one isomer may be outside of the coordination
sphere in the other isomer. The oxidation state of the central ion would not be
changed in the two isomers (Figure 7.3).
[Co(H2O)5Br]Cl [Co(H2O)5Cl]Br
Pentaaquabromocobalt(II) chloride Pentaaquachorocobalt(II) bromide
(I) (II)
Figure 7.3: Ionization isomerism

In isomer (I), the species generated from ionization are Cl- and [Co(H2O)5Br]+ while
in isomer (II), the ionized species are Br- and [Co(H2O)5Cl]+. The other ionization
isomers are:
One isomer [Co(NH3)5Br]SO4 (Pentaamminebromocobalt(II) sulphate) gives SO42-

ions in solution and gives white precipitate with BaCl2 while the another isomer
[Co(NH3)5 SO4]Br give Br- ions in solution and gives light yellow precipitate with
AgNO3. [Pt(NH3)3Br]NO2 (Triamminebromoplatinum(II) nitrite) and
[Pt(NH3)3(NO2)]Br (Triamminenitritoplatinum(II) bromide) are ionization isomers
and produce NO2- and Br- ions, respectively in solution.
7.3.1.3 Coordination isomerism

The coordination compounds in which both the cationic and anionic species
are complex ions, show coordination isomerism. This isomerism occurs by the
interchange of ligands in between the cationic part and the anionic part. In another
words, you can define the coordination isomers as the isomers in which the complex
anion and complex cation of a coordination compound exchange one or more ligands
(Figure 7.4).
[Co(NH3)6] [Cr(C2O4)3] [Co(C2O4)3] [Cr(NH3)6]
Hexaamminecobalt(III)
Hexaamminechromium(III)
tris(oxalato)chromate(III) tris(oxalato)cobaltate(III)
(A) (B)
Figure 7.4: Coordination isomerism
Another example of coordination isomers is [Zn(NH3)4][CuCl4] and

[Cu(NH3)4][ZnCl4].
7.3.1.4 Hydrate isomerism
In hydrate isomerism, there is exchange between water molecule inside the

coordination sphere and ions present in the ionisation sphere. Composition of hydrate
isomers is the same but the number of water molecules inside the coordination sphere
is different (Figure 7.5). For example, CrCl3.6H2O has the following three hydrate
isomers:
A. [Cr(H2O)6]Cl3 Hexaaquachromium(III) chloride (violet)
B. [Cr(H2O)5Cl]Cl2.H2O-Pentaaquachlorochromium(III) chloride monohydrate (blue

green)
C. [Cr(H2O)4Cl2]Cl.2H2O Tetraaquadichlorochromium(III) chloride dihydrate
(green)
Figure 7.5: Hydrate isomerism
These isomers have different chemical properties and on reaction with AgNO3, they
produce 3, 2 and 1 mole of AgCl suggesting 3, 2 and 1 Cl- (chloride ions) in solution,
respectively.
7.3.1.5 Ligand isomerism

Certain ligands such as diamine derivatives of propane show two isomeric
forms namely propylene diamine (pn) or 1, 2-diaminopropane and trimethylene
diamine (tn) or 1, 3-diaminopropane. When such type of ligands forms complexes
with metals, two isomers are obtained. The phenomenon is known as ligand
isomerism and the isomers are called as ligand isomers (Figure 7.6).
H2 H2
H2C C CH3 H2C C CH2
NH2 NH2 NH2 NH2
1,2-diaminopropane or 1,3-diaminopropane or
Propylenediamine (pn) Trimethylenediamine (tn)
(A) (B)
(A) and (B) are two isomeric ligands that show ligand isomerism.
+
pn +
N tn N
N N N N
Co3+ Co3+
pn tn
Cl N Cl N
Cl Cl
[Co(pn)2Cl2]+ [Co(tn)2Cl2]+
Figure 7.6: Ligand isomerism

7.3.1.6 Coordination position isomerism

This type of isomerism occurs in bridge compounds and arises due to differently
placed non-bridging ligands round the central metal atom/ion (Figure 7.7).
2+
OH
Co NH3 Cl2
NH3 Co
4 2
OH
Tetraamminecobalt(III)-µ-dihydroxodiamminedichlorocobalt(III)
2+
OH
Co NH3 Cl
Cl NH3 Co
3 3
OH
Triamminechlorocobalt (III)-µ-dihydroxotriamminechlorocobalt (III) ion
Figure 7.7: Coordination position isomerism
Ammonia and chloride ligands are differently placed in the above two different
isomers.
7.3.2 Methods for identification of structural isomers

(A) Conductivity method (Ionization isomers and hydrate isomers)
(B) Electrolysis method (Coordination isomers)
(C) Freezing point depression method (Hydrate isomers)
(D) Infrared spectroscopy (Linkage isomers)
7.3.3 Stereoisomerism
Compounds which have the same atoms/groups, same position of

atoms/groups and same sets of bonds, but differ in their spatial arrangement around
the central atom/ ion are called as stereoisomers and the phenomenon is known as
stereoisomerism. It is also called as space isomerism. It is of two types: Geometrical
isomerism and Optical isomerism. Complexes of Co(III) were the first known
coordination compounds to exist as stereoiomers. Purple and green salts of
[CoCl2(en)2]+ were first observed by Jorgensen in 1889, which were later identified as

the cis- and trans- isomers by Werner. Werner and King (1911) reported optical
isomers of the complex cis-[CoX(NH3)(en)2]2+ (where X=Cl- or Br-) for the first time.
7.3.3.1. Geometrical isomerism
The compounds with differences in geometrical arrangement of the ligands

around the central atom/ ion are known as geometrical isomers and the phenomenon
as geometrical isomerism. This is also called as cis-trans isomerism. The
geometrical isomers have the same empirical formula but different physical and
chemical properties due to different arrangement of the ligands in space. The
geometrical isomers can be easily separated from each other. When similar atoms/
groups (ligands) are adjacent to each other, the isomer is called cis-isomer (Latin, cis
= same). In trans-isomer, the similar ligands are present diagonally opposite to each
other (Latin, trans = across). It is common in di-substituted square planar and
octahedral complexes with co-ordination number of 4 and 6, respectively. Tetrahedral
(coordination number 4) complexes do not show geometrical isomerism because in
this geometry, all the ligands are present in cis- position (adjacent) with respect to
each other (all bond angles are same).
7.3.3.1.1 Four-coordinated compounds (Square planar complexes)

Among four coordinated complexes, square planar complexes show
geometrical isomerism. The four coordinated complexes that show/ do not show
geometrical isomerism are given in Figure 7.8.
[Ma4]n±, [Ma3b]n±, [Mab3]n±, “a”

and “b” are monodentate
Square planar complexes ligands
(Coordination number 4)
[Ma2b2]n±, [Ma2bc] n±,
[Mabcd]n±, where “a” and “b”
are monodentate ligands.
[M(AA)2] n±, [M(AB)2] n±, where

“AA” and “AB” are
symmetrical and
unsymmetrical bidentate
ligands respectively.
Figure 7.8: Geometrical isomerism in square planar complexes
(A) Compounds of the type [Ma2b2]n±
The compound with molecular formula [Pt(NH3)2(Cl)2], exists as two isomers: cis-
and trans -. cis- isomer is called as cis-platin and the trans- isomer as trans-platin.

Both the isomers have different chemical and biological properties. cis-
[Pt(NH3)2Cl2] is used as an anti-cancer agent in chemotherapy (cisplatin) while
the trans- isomer is inactive against cancer. cis- and trans- is the position of 2 atoms/
groups relative to each other (Figure 7.9). In the cis- isomer, two atoms/ groups are
at 90º angle with respect to the central metal atom/ ion or adjacent to each other,
whereas in the trans- isomer, the atoms/ groups are at 180º angle with respect to the
central metal atom/ ion or "opposite to each other". In the complex of type [Ma2b2]n±,
both the cis- and trans- isomers are cis and trans with respect to the position of both
the ligands “a” and “b”. Only two geometric isomers are possible for this type of
compound. The another example is [Pd(NH3)2(NO2)2]:
n± n±
a b a b
M M
a b b a
cis-[Pt(NH3)2(Cl)2] trans-[Pt(NH3)2(Cl)2]
cis-diamminedichloroplatinum(II) trans-
diamminedichloroplatinum(II)
a=Cl; b=NH3; M= Pt(II)
Figure 7.9: cis- and trans- isomers of [Ma2b2]n± type compound
(B) Compounds of the type [Ma2bc]n±
In this type of compounds, cis- and trans- isomers are found with respect to the
similar atoms/ groups such as “a”. Atom/ group “a” is a neutral ligand such as NH3,
py and H2O while “b” and “c” are anionic ligands like Cl-, Br-, NO2- etc (Figure
7.10).
n± n±
a b a b
M M
a c c a
a=NH3; b= Cl-, c=Br-; M= Pt (II); n± = 0
Figure 7.10: cis- and trans- isomers of [Ma2bc]n± type compounds

(C) Compounds of the type [Mabcd]n±
Compounds of type [Ma2bc]n± have three geometrical isomers. Il'ya Chernyaev

(1928) had given the first report on the three geometric isomers for complexes of the
type [Mabcd]n±. He isolated and characterized all the three isomers. The three isomers
can be obtained by fixing one atom/ group and the other atoms/ groups in trans to the
fixed one. Platinum (Pt) shows a number of compounds of this type (Figure 7.11).
n± n± n± n±
a b a c a b a c
M M M M
d c d b c d b d
(I) (II) (III) (IV)
a=NH3; b= Cl-; c=Br-; d=py (C5H5N) or C2H4; M= Pt(II); n± = 0
Figure 7.11: Geometrical isomers of compounds of the type [Mabcd]n±
Structure III and IV are similar as b and c are next to “a” in each. Therefore, overall
three isomers (I, II and III~IV) may exist in which c, b and d are trans to “a”.
(D) Compounds of the type [M(AA)2]n±
Symmetrical bidentate ligands such as NH2.CH(CH3).CH(CH3).NH2 with platinum

metal ion show cis-trans isomerism (Figure 7.12).
2+
H3C CH3 2+
C H2N C H CH3
NH2 C
H H H2 N NH2 C
Pt H 3C
Pt H
H3 C C H2N NH2 C CH3 H C H2N NH2 C
CH3
H H H3 C H
cis-isomer trans-isomer
Figure 7.12: Geometrical isomerism in compounds of the type [M(AA)2]n±
(E) Compounds of the type [M(AB)2]n±
When the similar atoms (A) of both the unsymmetrical bidentate ligands are near to
each other (at 90º angle with respect to central metal), the isomer is cis- and if the
similar atoms present at opposite position to each other, the isomer will be trans-
(Figure 7.13).

n± n±
A A A B
M M
B B B A
trans-isomer
AB = gly (NH2CH2COO-); M = Pt(II); n± = 0
Figure 7.13: Geometrical isomerism in compounds of the type [M(AB)2]n±
7.3.3.1.2 Six-coordinated compounds (octahedral complexes)

The types of six-coordinated compounds that can show / or cannot show geometrical
isomerism, are presented in Figure 7.14.
[Ma5b]n±, [Mab5]n±, [Ma6]n±,

“a” and “b” are monodentate
Octahedral complexes ligands
(Coordination number
6) [Ma4b2]n±, [Ma3b3] n±,
[Mabcdef]n±, where “a” and
“b” are monodentate ligands.
[M(AA)2a2] n±, [M(AA)2ab] n±

[M(AA)a2b2] n± , [M(AB)3] n±,
where “a” and “b” are
monodentate ligands while
“AA” and “AB” are
symmetrical and
ligands respectively.
Figure 7.14: Geometrical isomerism in octahedral complexes
(A) Compounds of the type [Ma4b2]n±
Two isomers are possible for such compounds: cis- and trans-. The two similar atoms
or groups are at 90º angle with respect to the central metal atom / ion (Figure 7.15).

b b
n± n±
a b a a
M M
a a a a
a cis-isomer b trans-isomer
a=NH3; b= Cl-, M= Co; n± = +1
Figure 7.15: Geometrical isomerism in compounds of the type [Ma4b2]n±
(B) Compounds of the type [Ma3b3]n±
In case of [Ma3b3]n± type compounds, the two possible geometrical isomers are called
as facial and meridional isomers. When the three same atoms or groups are placed on
one face of the octahedral, facial isomer is generated. The three similar atoms or
groups placed in a plane passing through metal aotm/ ion around the centre gives
rise to another isomer called as meridional isomer. The isomers are named on the basis
of relative positions of the ligands around the octrahedron (Figure 7.16).
a a n±
n±
b a
b b
M M
b a b a
b a
(1, 2, 3)-isomer (1, 2, 6)-isomer
facial (fac-) isomer meridional (mer-) isomer
a=py; b= Cl-, M= Rh(III); n± = 0
Figure 7.16: Geometrical isomerism in compounds of the type [Ma3b3]n±
(C) Compounds of the type [Mabcdef]n±
This type of compounds may give rise to 15 isomers obtained after exchanging all the
ligands (Figure 7.17). [Pt(py)(NH3)(NO2)(Cl)(Br)(I)] is the only compound that
shows this type of geometrical isomerism.

a n±
f b
M
e c
b n± c n± d n± e n± f n±
f a f b f b f b a b
M M M M M
e c e a e c a c e c
d d a d d
Arises due to exchange of "a" with the other five (b,c,d,e,f) groups or atoms (five isomers)
a n± a a a n± a
n± n± n±
f b
f f f e b f
c d
M M
M M M
e c b
e b e c c e c
d d
d b d
Arises due to exchange of "b" with the other four (c,d,e,f) groups or atoms (four isomers)
a n± a n± a n± a n±
f b f b f b c b
M M M M
e c e d c e e f
d c d d
Arises due to exchange of "c" with the other three (d,e,f) groups or atoms three isomers)
a n± a a
n± n±
f b f b d b
M M M
e c d c e c
d e f
Arises due to exchange of "d" with the other two (e,f) groups or atoms (two isomers)
a n± a n±
f b e b
M M
e c f c
d d
Arises due to exchange of "e" with the f (one isomers)
Figure 7.16: Fifteen geometrical isomers of compound [Mabcdef]n±
(D) Compounds of the type [M(AA)2a2]n±
Two geometrical isomers are possible for [M(AA)2a2]n± type of compounds. In cis-
form monodentate ligands, “a”s are cis to each other while in trans-isomer, “a”s are
trans to each other (Figure 7.17).

n± n±
A a
a A A A
M M
a A A A
A a
AA = en; a = Cl-, M = Co(III); n± = +1
Figure 7.17: Geometrical isomerism in compounds of the type [M(AA)2a2]n±
(E) Compounds of the type [M(AA)a2b2]n±
[Co(en)(NH3)2Cl2]+ Compound shows geometrical isomerism of this type (Figure

7.18).
n± n±
a a
a A b A
M M
b A b A
b a
AA=en; a= Cl-; b=NH3; M= Co(III); n± = +1
Figure 7.18: Geometrical isomerism in compounds of the type [M(AA)a2b2]n±
(F) Compounds of the type [M(AB)3]n±
One example of this type of compound is trisglycinatochromium (III), [Cr(gly)3],

(Figure 7.19).
n± n±
B A
A A B B
M M
B B B A
A A
AB=gly (NH2CH2COO-); M= Cr (III); n± = 0
Figure 7.19: Geometrical isomerism in compounds of the type [M(AB)3]n±

7.3.3.2 Optical isomerism

Certain complex compounds in solution form can rotate the plane of polarized
light either in right or left direction. This property of a compound to rotate plane of
polarized light is called optical activity and the compounds which possess this
property are called as optically active compounds. The compound that can rotate
plane of polarized light towards right side are called as dextrorotatory (d of +) while
the compound that can rotate the plane of polarized light towards left side is called as
laevorotatory (l or -). These optically active compounds are also called as optical
isomers or enantiomorphs or enantiomers (Latin, enantio=opposite;
morphs=forms). Optical isomers have same physical and chemical properties but
have different property with respect to the rotation of the plane of polarized light and
the isomerism possessed by these isomers is called as optical isomerism or
enantiomerism or chirality. The compounds that cannot rotate plane of polarized
light are called as optically inactive compounds or racemic mixture (dl or ± form;
50% d form and 50% l form). The compounds that show optical isomerism should be:
- Asymmetric (no plane of symmetry). The molecules cannot be divided into

two equal parts when cut through an imaginary plane (mirror plane) (Figure
7.20). You can also call them chiral compounds.
Mirror plane Mirror plane
Symmetric having plane of symmetry Asymmetric having no plane of

symmetry
Figure 7.20: Examples showing symmetry and asymmetry
The mirror images are non-superimposable on each other (Figure 7.21).
Mirror plane Non-superimposable
Figure 7.21: Examples showing nonsuperimposable mirror images

7.3.3.2.1 Four-coordinate complexes
You should remember that tetrahedral and square planar complexes such as [Ma4]n±,
[Ma3b]n± and [Mab3]n± type do not possesses optical isomerism because all possible
arrangement of bonds around the central metal atom or ion are identical (Figure 7.22).
Square planar complexes

with formula [Ma4]n±,
[Ma3b]n±, [Mab3]n±, “a” and
Complexes with “b” are monodentate ligands
coordination number 4
Square planar complexes

with iso-butylenediammine
and meso-stilbenediammine
ligands.
Tetrahedral complexes of
[Mabcd] n± type and
[M(AB)2]n± type where a, b, c
and d are monodentate
ligands while AB is an
ligand.
Figure 7.22: Optical isomerism in four coordinate complexes
(A) Square planar complexes

Optical isomerism is rare in case of square planar complexes even if all the four
ligands are different. In a complex of Pt(II) with iso-butylenediammine and meso-
stilbenediammine ligands, optical isomerism was observed. This is so because, both
the phenyl groups of meso-stilbenediammine ligand are present above the plane of
the ring and H atoms below the plane of the ring. Therefore, the compound has no
plane of symmetry and possesses optical isomerism. Mills and Quibell (1935)
resolved the two optical isomers of this compound (Figure 7.23).

2+ 2+
H H
H H2 H2 H
H2 H2
C N N C C N N C
C6H5
H
(I) Pt (II) H
C6H5 (II)
H Pt (I) H
CH3 CH3
C N N C H N
H2 H2 N C
H2 C6H5 H2
CH3 C6H5
CH3
Mirror plane
(I) = iso-butylene diammine; (II) = meso-diphenyl ethylene (stilbene) diammine
Figure 7.23: Square planar complex showing optical isomerism
(B) Tetrahedral complexes
• Tetrahedral complexes such as [Mabcd]n± show optical isomerism (Figure
7.24).
n± n±
a a
M M
d b b d
c c
Mirror plane
a = CH3; b = C2H5; c = S2-; d = C6H4COO]2+; M = As3+
Figure 7.24: Optical isomerism in [Mabcd]n± type complexes
• Complexes of the [M(AB)2]n± type: Tetrahedral complexes of some metals

like Be(II), B(II) and Zn(II) with unsymmetrical ligands such as
benzoylacetone possess optical isomerism (Figure 7.25).
H3C CH3 H3C CH3
C O O C C O O C
HC Be CH HC Be CH
C O O C C O O C
C6H5 C6H5 C6H5 C6H5
Mirror plane
Figure 7.25: Optical isomerism in tetrahedral complex with unsymmetrical
bidentate ligand
7.3.3.2.2 Six-coordinate complexes

Octahedral complexes (six-coordinate complexes) that can show optical

isomerism are given in Figure 7.26. The details of optical isomerism of six-
coordinate compounds are further given in this section.
[Ma5b]n±, [Mab5]n±, [Ma6]n±, “a”

and “b” are monodentate
Complexes with ligands
coordination number 6
[Ma2b2c2]n±; [Ma2b2cd] n±;

[Ma2bcde] n±; [Mabcdef]n±;
where “a” and “b” are
monodentate ligands.
[M(AA)3] n±; [M(AA)2BB] n± ;

[M(AA)2a2] n± ; [M(AA)2ab];
[M(AA)a2b2] n±, where “a” and
“b” are monodentate ligands
while “AA” and “AB” are
symmetrical and
ligands, respectively.
Figure 7.26: Optical isomerism in six coordinate complexes
(A) Compounds of the type [Ma2b2c2]n± (Figure 7.27)
b b n±
n±
a b b a
M M
a c c a
c c
Mirror plane
Figure 7.27: Optical isomerism in [Ma2b2c2]n± type compounds
(B) Compounds of the type [Ma2b2cd]n± (Figure 7.28)

b b n±
n±
a b b a
M M
a d d a
c c
Mirror plane
Figure 7.28: Optical isomerism in [Ma2b2cd]n± type compounds
(C) Compounds of the type [Ma2bcde]n± (Figure 7.29)
b b n±
n±
a e e a
M M
a d d a
c c
n±
Figure 7.29: Optical isomerism in [Ma2bcde] type compound
(D) Compounds of the type [Mabcdef]n±
Total thirty optical isomers are possible for this compound. Each of fifteen
geometrical isomers has two enantiomers (Figure 7.30). The example of this type of
complex is [Pt(py)(NH3)(NO3)(Cl)(Br)(I)].
e e n±
n±
a d d a
M M
b c c b
f f
Mirror plane
Figure 7.30: Optical isomerism in [Mabcdef]n± type compounds
(E) Compounds of the type [M(AA)3]n±
A number of coordination compounds of this type show optical isomerism (Figure
7.31). Metals like Co(III), Pt(IV), Fe(III), Ir(IV), Rh(IV), Co(II and III), Al(III) and
Zn (II) with symmetrical bidentate ligands such as ethylene diamine (en), oxalato and
propylene diamine (pn) show optical isomerism.
A A
n± n±
A A A A
M M
A A A A
A A
Mirror plane
Figure 7.31: Optical isomerism in [M(AA)3]n± type compounds

(F) Compounds of the type [M(AA)2(BB)]n±
In this type of compounds, AA and BB are two different kinds of symmetrical

bidentate ligands. The ligand AA is neutral symmetrical bidentate ligand while BB is
anionic symmetrical bidentate ligand (Figure 7.32).
A A
n± n±
B A A B
M M
B A A B
A A
Mirror plane
AA = en; BB = C2O42-; M = Co(III); n± = +1
Figure 7.32: Optical isomerism in [M(AA)2(BB)]n± type compounds
(G) Compounds of the type [M(AA)2(a)2]n±
These types of compounds have two geometrical isomers: cis- and trans-. The trans-
isomer (Figure 7.34) is mesomeric (optically inactive having plane of symmetry) in
nature while the cis-form exists as two optical isomers. These two forms are mirror
image of each other (Figure 7.33).
A A
n± n±
a A A a
M M
a A A a
A A
Mirror plane
AA = en; a = Cl-; M = Co(III); n± = +1
Figure 7.33: Optical isomerism in [M(AA)2(a)2]n± type compounds (cis-isomer

with respect to “a”)

a
n±
A A
M
A A
Mirror plane
Figure 7.34: trans-isomer of [M(AA)2(a)2]n± complex which is mesomeric in nature
(H) Compounds of the type [M(AA)2ab]n±
This type of compound exists in three isomeric forms. Two cis- forms (Figure 7.35)
are optically active while the trans- form is optically inactive (meso- form) (Figure
7.36).
a a
n± n±
b A A b
M M
A A A A
A A
Mirror plane
AA = en; a = NH3; b = Cl-; M = Co(III); n± =+2
Figure 7.35: Optical isomerism in cis-[M(AA)2ab]n± type compounds
a
n±
A A
M
A A
Mirror plane
Figure 7.36: meso form of [M(AA)2ab]n± type compound
(I) Compounds of the type [M(AA)a2b2]n±
This type of compounds also exist in three isomeric forms: two optically active cis-
forms (Figure 7.37) and one optically inactive trans-form (meso form with plane of
symmetry) (Figure 7.38).
a a n±
n±
a A A a
M M
b A A b
b b
Mirror plane
AA = C2O42-; a = NH3; b = NO2-; Co (III); n± =+1
Figure 7.37: Optically active form of cis- [M(AA)a2b2]n± type compound
a
n±
b A
M
b A
Figure 7.38: meso form of trans-[M(AA)a2b2]n± compound
7.3.3.3 Importance of stereoisomerism
The chemistry of our body is controlled by stereochemically active molecules

and therefore, different stereoisomers of a molecule react in different ways in our
body system. Different enantiomers of chiral drug exhibit different reactivity. Only
one chiral form of chiral molecules is present in living organisms. The amino acids
present in proteins are found as their L iosmers whereas glucose occurs as its D
isomer in human body. Most of the drugs are generally composed of a single
stereoisomer of a compound. One stereoisomer may positively affect our body or the
other isomer may have toxic effects. Therefore, it is a challenge today to synthesize
stereochemically pure drugs. S-enantiomer of citalopram; an antidepression drug, is
thirty times potent as comparison to R-enantiomer. Thalidomide, a very common
drug used for treatment of morning sickness in pregnant women in 1950s-1960s, was
found to be a racemic mixture (mixture of the two isomers: R- and S- which are
mirror image to each other) caused serious birth defects. R- isomer can treat morning
sickness while the other isomer S- causes birth defects. Even if the correct isomer is

given for treatment, it underges racemisation which results in the production of both
safe and harmful forms of thalidomide.
Cisplatin or cis-platinum, or cis-diamminedichloroplatinum (II) (CDDP), an
inorganic stereoisomer, is a chemotherapy drug used for treatment of ovarian and
testicular cancers while trans-platin, the trans stereoisomer of cis-platin is toxic in
nature.
Cl Cl
Cl Pt NH3 H3N Pt NH3
H3N Cl
cis-diamminedichloroplatinum (II) trans-diamminedichloroplatinum (II)

(cis-platin) (trans-platin)
The activity of a particular isomer is due to the stereospecific (the ‘fit’ in such must
be correct as a lock and key fit) nature of biological interactions between molecules.
7.3.3.4 Methods for identification of stereoisomers
(A) Dipole moment method (cis and trans isomers)
(B) Nuclear Magnetic Resonance (NMR) Spectroscopy (cis and trans

isomers)
(C) Greinberg’s method (cis and trans isomers)
(D) Mass spectrometry (both optical and cis and trans isomers)
(E) Infrared Spectroscopy (cis and trans isomers)
7.4 VALENCE BOND THEORY (VBT) OF TRANSITION

METAL COMPLEXES
Valence bond theory was given by Pauling and Slater in 1935. According to this
theory:
In coordination compounds, the ligands form covalent-coordinate bonds to the
metal atom/ ion. The central metal atom/ ion provide vacant orbitals (s, p and
/or d atomic orbitals) equal to its coordination number. These vacant orbitals

hybridize and form the same number of new hybridized orbitals (atomic
orbitals overlap) of equal energy.
Ligands can donate at least one lone pair (in σ orbital) of electrons to the
empty hybrid orbitals of the central metal atom/ ion.
Each ligand with filled σ orbital then overlap with the empty hybrid orbital of
central metal atom/ ion.
This theory helps in predicting the shape, stability and calculating magnetic moment
(magnetic property: µ=√n(n+2) of the metal complexes.
Spectrochemical series of ligands
A spectrochemical series is the ordered arrangement on ligand strength from lower
strength (small ∆o) to higher strength (large ∆o). The ligands on the left end are
weaker ligands (σ-donor) which can donate a pair the electrons in (n-1)d orbitals of
metal/ion. The right end ligands are stronger (π-acceptor) ligands and can accept pair
of electrons from filled (n-1)d orbitals in the metal atom/ion. The spectrochemical
series of ligands is given as:
I− < Br− < S2− < SCN− < Cl− < NO3− < N3− < F− < OH− < C2O42− ≈ H2O < NCS− <
CH3CN < py < NH3 <en < phen < NO2− < PPh3 < CN− ≈ CO
The position of ligand in the spectrochemical series and nature of central metal atom/
ion affects the hybridization, structure, stability and magnetic moment of complexes.
(A) Four coordinate compounds (Tetrahedral complexes)
In case of tetrahedral complexes, the central metal atom / ion provides four vacant
orbitals (one s and three p). These four orbitals hybridize to form four sp3 hybridized
orbials. Each hybridized orbital then overlaps with the filled p orbitals of the each
ligand. The geometry of such complexes will be tetrahedral. Hybridization and thus,
geometry of a complex depends upon the type of ligand (σ-donor or π-acceptor)
attached to the central metal atom / ion. Electrons of metal atom / ion are shown as ↑↓
, whereas those of the electron pair on ligand are depicted as xx. For example,
a. [ZnCl4]-2: Zn = 3d104s2 and Zn+2 = 3d10
In this complex, the ligand is weak and the d orbitals of Zn2+ ion are filled. Hence,
it is a diamagnetic substance (no unpaired electron) and the geometry is
tetrahedral. The central metal ion; Zn2+ is sp3 hybridized.

2-
Cl
Ni
INORGANIC CHEMISTRY-II
Cl Cl BSCCH-201
Cl
3d 4s 4p
Zn
4sp3 hybridized orbitals

3d 4s 4p
Zn2+
3d 4s 4p
2-
[ZnCl4] xx xx xx xx
Cl- Cl- Cl- Cl-
Ni
b. [NiCl4]2- : Electronic configuration of is 3d84s2 and hence, electronic
configuration of Ni+2 is 3d8.
In this complex, the ligand is weak and no pairing of electrons will occur. There
are two unpaired electrons in d orbitals of Ni2+. Hence, it is a paramagnetic in
nature. The central metal ion; Ni2+ is sp3 hybridized and the geometry is
tetrahedral.
3d 4s 4p
Ni

3d 4s 4p
Ni2+
3d 4s 4p
2-
[NiCl4] xx xx xx xx
Cl- Cl- Cl- Cl-
2-
Cl
Zn
Cl Cl
Cl

c. [Ni(CO)4] : Ni = 3d8 4s2 . In this complex, Ni is in 0 oxidation state. The ligand is

strong and hence, pairing of electrons will occur. There are no unpaired electrons
in d orbitals of Ni2+. Hence, it is a diamagnetic in nature. The central metal atom
Ni is sp3 hybridized and the geometry is tetrahedral.
3d 4s 4p
Ni

3d 4s 4p
[Ni(CO)4] xx xx xx xx
-
CN- CN- CN- CN
O
C
Ni
OC CO
C
O
d. [MnCl4]2- : Electronic configuration of Mn (atomic number 25) is 3d5 4s2,

and hence, the electronic configuration of Mn+2 is 3d5. In this complex, again
the ligand is weak and no pairing of electrons will occur. There are five unpaired
electrons in d orbitals of Mn2+. Hence, it is a paramagnetic in nature. The central
metal ion; Mn2+ is sp3 hybridized and the geometry is tetrahedral.
3d 4s 4p
Mn

3d 4s 4p
Mn2+
3d 4s 4p
[MnCl4]2- xx xx xx xx
Cl- Cl- Cl- Cl-

2-
Cl
Mn
Cl Cl
Cl
(B) Four coordinate compounds (Square planar complexes)
a. [Ni(CN)4]2- : As you know, the electronic configuration of Ni = 3d8 4s2 , nickel

in this complex is in +2 oxidation state, hence Ni+2 = 3d8. You also know that
CN- is a strong field ligand, hence pairing of electrons will occur. There will be
no unpaired electrons in d orbitals of Ni2+. Hence, it is diamagnetic in nature.
The central metal ion; Ni2+ provides one inner d orbital and thus, is dsp2
hybridized and square planar.
3d 4s 4p
Ni
3d 4s 4p
Ni2+
4dsp2 hybridized orbitals

3d 4s 4p
[Ni(CN)4]2- xx xx xx xx
CN- CN- CN- CN-
2-
- -
CN CN
Ni
CN- CN-
In 'dsp2' hybridization,’d’ preceeds 'sp2' indicating that the lower (inner) shell d-
orbital is used in hybridization that comes before the 's' and 'p' orbitals. This is called
inner shell hybridization (form inner shell complexes). When outer shell d-orbitals
are used in hybridization, it is called as outer shell (form outer shell complexes)
hybridization.
(C) Six coordinate compounds (Octahedral complexes)

a. [Cr (NH3)6]3+: Chromium (atomic no. 24) has 3d54s1 as valence shell
configuration and is in +3 oxidation state in this complex. Cr+3 thus has 3d3
configuration.
In this complex, although NH3 is a strong field ligand, but no pairing of
electrons is required. There are three unpaired electrons in d orbitals of Cr3+.
Hence, it is a paramagnetic in nature. The central metal ion; Cr3+ provides
inner d orbitals and thus, is d2sp3 hybridized (inner shell orbital complex) and
octahedral in shape.
3d 4s 4p
Cr
6dsp3 hybridized orbitals

3d 4s 4p
3+
Cr
3d 4s 4p
3-
[Cr(NH3)6] xx xx xx xx xx xx
NH3 NH3 NH3 NH3 NH3 NH3
3+
H3
N
NH3
H3N Cr NH 3
H3N
N
H3
b. [CoF6]3- : Cobalt (atomic no. 27) has 3d7 4s2 configuration . In this complex
cobaly is in +3 oxidation state and thus the electronic configuration of Co+3 is
3d6. In this complex, the ligands are weak, so no pairing of electrons will
occur. There are four unpaired electrons in d orbitals of Co3+. Hence, it is
paramagnetic in nature. The central metal ion Co3+ provides outer d orbitals
and thus, is sp3d2 hybridized (outer shell orbital complex) and octahedral in
shape.

3d 4s 4p
Co
3d 4s 4p
3+
Co
6sp3d2 hybridized orbitals

3d 4s 4d
4p
3-
[Co(F)6] xx xx xx xx xx xx
F- F- F- F- F- F-
c. [Co(NH3)6]3+: In this complex also, cobalt is in +3 oxidation state and has

3d6 configuration. As NH3 is a strong field ligand, pairing of electrons will
occur. There are no unpaired electrons in d orbitals of Co3+. Hence, it is a
diamagnetic in nature. The central metal ion Co3+ provides inner d orbitals
for complex formation, is thus d2sp3 hybridized (inner shell orbital complex)
and the complex is octahedral in shape:
3d 4s 4p
Co
3d 4s 4p
3+
Co
6d2sp3 hybridized orbitals

3d 4s 4p
[Co(NH3)6]3- xx xx xx xx xx xx
NH NH NH NH NH NH
3 3 3 3 3 3
d. [Fe(CN)6]3- : Iron (atomic no. 26) has 3d6 4s2 as valence shell configuration,
with iron in +3 oxidation state; Fe+3 having 3d5 configuration. In this
-
complex, the ligand CN is a strong field ligand, hence, pairing of electrons
will takes place. There will be one unpaired electron in d orbital of Fe3+.
Hence, it will be paramagnetic in nature. The central metal ion, Fe3+, provides

inner d orbitals and thus, is d2sp3 hybridized (inner shell orbital complex) and
3d 4s 4p
Fe
3d 4s 4p
Fe3+

3d 4s 4p
[Fe(CN)6]3- xx xx xx xx xx xx
CN- CN- CN- CN- CN- CN-
e. [Fe(H2O)6]3+ : Iron (atomic no. 26) has 3d6 4s2 as valence shell
configuration, with iron in +3 oxidation state; Fe+3 having 3d5 configuration.
In this complex, the ligand water is a weak field ligand hence; pairing of
electrons does not take place. There are five unpaired electrons in d orbitals of
Fe3+. Hence, it is paramagnetic in nature. The central metal ion Fe3+ provides
outer d orbitals and is thus, sp3d2 hybridized (outer shell orbital complex) and
3d 4s 4p
Fe
3d 4s 4p
3+
Fe
6 sp3d2 hybridized orbitals

3d 4s 4p 4d
[Fe(H2O)6]3+ xx xx xx xx xx xx
H2O H2O H2O H2O H2O H2O
f. [Mn(CN)6]4- : Manganese (atomic no. 25) has 3d54s2 as valence shell

configuration, with manganese in +2 oxidation state; Mn2+ having 3d5
configuration. In this complex, the CN- ligand is a strong field ligand. Hence,

pairing of electrons takes place. However, there is one unpaired electron in d

orbitals of Mn2+, it is a paramagnetic in nature. The central metal ion Mn2+
provides inner d orbitals and is d2sp3 hybridized (inner shell orbital complex).
Therefore, the complex, [Mn(CN)6]4- has an octahedral structure:
3d 4s 4p
Mn
3d 4s 4p
2+
Mn

3d 4s 4p
[Mn(CN)6]4- xx xx xx xx xx xx
- -
CN- CN- CN- CN- CN CN
Limitations of VBT:
Cannot explain colour of complexes.
Cannot explain why magnetic moments of some metal complexes are
temperature dependent.
Cannot explain the structure of Cu2+ complexes.
7.5 SUMMARY
In this unit, you have studied that:
• Isomers are the molecules having the same number of atoms/ groups and
thus, same chemical formula but have different structural formula
(different arrangement of atoms or groups).
• Isomers can be divided into two main categories: structural isomers and
space or stereoisomers.
• In structural isomers, the atoms/ groups are arranged in different ways
(pattern of bonding is different).
• In stereoisomers, the arrangement of atoms/ groups in space is different
and their pattern of bond is the same. These are of two types: geometrical
isomerism and optical isomerism.

Coordination 1

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INORGANIC CHEMISTRY-II BSCCH-201

• Applications of coordination compounds,

6.3.2 Ligands and their types

6.2.1 Types of ligands

The ligands can be classified in the following ways:

6.2.2.1 Type I- Based on electron accepter/donor properties of the ligand

UTTARAKHAND OPEN UNIVERSITY Page 97

Figure 6.1: Classification of monodentate ligands

• Polydentate (bidentate, tridentate, tetradentate, pentadentate,

UTTARAKHAND OPEN UNIVERSITY Page 98

Table 6.1: Polydentate ligands

Ligand type Ligand name Ligand formula/structure

Bidentate oxalate ion O-

UTTARAKHAND OPEN UNIVERSITY Page 99

Tridentate diethylenetriamine (H2C)2

Tetradentate triethylenetetramine NH2(CH2)2NH(CH2)2NH(CH2)2NH2

(EDTA) -O C CH2 H2C C O-

6.2.2.3 Type III- Based on size of ligand

UTTARAKHAND OPEN UNIVERSITY Page 100

rigid and inert as compared to the chelate compound and is known as

Figure 6.2: Nickel(II) bisdimethylglyoximate

The order of ligands to form stable compounds

Macrocyclic > Chelate > Monodentate

6.2.2.4 Type IV- Based on their use in reactions

• Spectator ligands are tightly coordinating polydentate liangds

6.2.2 Complex ion

6.2.3 Coordination number (CN)

UTTARAKHAND OPEN UNIVERSITY Page 101

The coordination number of a metal atom/ion is the number of ligands attached

– large metal atoms show high CN

Table 6.2: Coordination numbers (CN) and geometry of coordination

2 4 Small, high Large Tetrahedral L

d8 metal atoms or Pi bonding Square L L

3 5 Allows fluxionality Trigonal L

UTTARAKHAND OPEN UNIVERSITY Page 102

6 8 Generally shown by For eight Dodecahed

Large metal ions

UTTARAKHAND OPEN UNIVERSITY Page 103

7 9 Very rare Three-face

AA 11 Very rare All-faced

6.2.4 Coordination sphere

The coordination sphere of a coordination compound comprises the central metal

UTTARAKHAND OPEN UNIVERSITY Page 104

Coordination sphere [Co(NH3)6]3+

6.2.5 Counter ions:

• The attachment of species that satisfy both primary as well as

NH3 Cl- Cl-

H3N NH3 H3N NH3 H3 N NH3

H3N NH3 H 3N NH3 H3 N NH3

(a) (b) (c) (d)

Figure 6.3 Structure of Co(III) amine complexes on the basis of Werner’s

Werner’s theory can be explained on the basis of experimental evidences

Compounds [Co(NH3)6]Cl3, [Co(NH3)5Cl]Cl2, [Co(NH3)4Cl2]Cl and

[Co(NH3)6]Cl3 [Co(NH3)6]3+ + 3Cl- (1 complex ion + 3 chloride

This rule is given by English Chemist Nevil V. Sidgwick. Effective atomic

UTTARAKHAND OPEN UNIVERSITY Page 106

EAN = 24 + (6 x 2) = 36 (atomic number of Krypton; Kr)

EAN = 28 + (4 x 2) = 36 (atomic number of Krypton; Kr)

EAN = 24 + (6 x 2) = 36 (atomic number of Krypton;