Transition Metal Complexes
Transition Metal Complexes
Transition Metal Complexes
TRANSITION METAL
COMPLEXES
St John’s College
Chemistry Department
TRANSITION METALS
CONTENTS
• Aqueous metal ions
• Acidity of hexaaqua ions
• Introduction to the reactions of complexes
• Reactions of cobalt
• Reactions of copper
• Reactions chromium
• Reactions on manganese
• Reactions of iron(II)
• Reactions of iron(III)
• Reactions of silver and vanadium
• Reactions of aluminium
THE AQUEOUS CHEMISTRY OF IONS - HYDROLYSIS
Acidity as charge density increases, the cation has a greater attraction for water
the attraction extends to the shared pair of electrons in water’s O-H bonds
the electron pair is pulled towards the O, making the bond more polar
this makes the H more acidic (more d+)
it can then be removed by solvent water molecules to form H 3O+(aq).
HYDROLYSIS - EQUATIONS
the resulting solution will now be acidic as there are more protons in the water
this reaction is known as hydrolysis - the water causes the substance to split up
Stronger bases (e.g. CO32- , NH3 and OH¯ ) can remove further protons...
HYDROLYSIS - EQUATIONS
the resulting solution will also be acidic as there are more protons in the water
this SOLUTION IS MORE ACIDIC due to the greater charge density of 3+ ions
Stronger bases (e.g. CO32- , NH3 and OH¯ ) can remove further protons...
HYDROLYSIS OF HEXAAQUA IONS
Lewis bases can attack the co-ordinated water molecules. Theoretically, a proton can be
removed from each water molecule turning the water from a neutral molecule to a
negatively charged hydroxide ion. This affects the overall charge on the complex ion.
OH¯ OH¯
[M(H2O)6]2+(aq) [M(OH)(H2O)5]+(aq) [M(OH)2(H2O)4](s)
H +
H +
OH¯ OH¯
[M(OH)2(H2O)4](s) [M(OH)3(H2O)3]¯(aq) [M(OH)4(H2O)2]2-(aq)
H +
H +
Lewis bases can attack the co-ordinated water molecules. Theoretically, a proton can be
removed from each water molecule turning the water from a neutral molecule to a
negatively charged hydroxide ion. This affects the overall charge on the complex ion.
OH¯ OH¯
[M(H2O)6]2+(aq) [M(OH)(H2O)5]+(aq) [M(OH)2(H2O)4](s)
H+
H+
Precipitated
OH¯ OH¯
[M(OH)2(H2O)4](s) [M(OH)3(H2O)3]¯(aq) [M(OH)4(H2O)2]2-(aq)
H+
H+
In some cases, if the base is strong, further protons are removed and the precipitate
dissolves as soluble anionic complexes such as [M(OH) 6]3- are formed.
Lewis bases can attack the co-ordinated water molecules. Theoretically, a proton can be
removed from each water molecule turning the water from a neutral molecule to a
negatively charged hydroxide ion. This affects the overall charge on the complex ion.
OH¯ OH¯
[M(H2O)6]2+(aq) [M(OH)(H2O)5]+(aq) [M(OH)2(H2O)4](s)
H+
H +
Precipitated
OH¯ OH¯
[M(OH)2(H2O)4](s) [M(OH)3(H2O)3]¯(aq) [M(OH)4(H2O)2]2-(aq)
H+
H +
AMPHOTERIC CHARACTER
Metal ions of 3+ charge have a high charge density and their hydroxides can
dissolve in both acid and alkali.
[M(H2O)6]3+(aq) H[M(OH)
+
OH¯
3(H2O)3](s) [M(OH)6]3-(aq)
The examples aim to show typical properties of transition metals and their compounds.
One typical properties of transition elements is their ability to form complex ions.
Complex ions consist of a central metal ion surrounded by co-ordinated ions or
molecules known as ligands. This can lead to changes in ...
Reaction
types ACID-BASE A-B
LIGAND SUBSTITUTION LS
PRECIPITATION Ppt
The examples aim to show typical properties of transition metals and their compounds.
LOOK FOR...
substitution reactions of complex ions
variation in oxidation state of transition metals
the effect of ligands on co-ordination number and shape
increased acidity of M3+ over M2+ due to the increased charge density
differences in reactivity of M3+ and M2+ ions with OH¯ and NH3
but ... ammonia ligands make the Co(II) state unstable. Air oxidises Co(II) to Co(III)
A-B
NH3 [Cu(H2O)6]2+(aq) + 2NH3(aq) ——> [Cu(OH)2(H2O)4](s) + 2NH4+(aq)
blue ppt. soluble in excess NH3
A-B
NH3 [Cu(H2O)6]2+(aq) + 2NH3(aq) ——> [Cu(OH)2(H2O)4](s) + 2NH4+(aq)
blue ppt. soluble in excess NH3
Ppt
CO3 2-
[Cu(H2O)6] (aq)2+
+ CO3 (aq)
2-
——> CuCO3(s) + 6H2O(l)
blue ppt.
REACTIONS OF COPPER(II)
Cl¯ [Cu(H2O)6]2+(aq) + 4Cl¯(aq) ——> [CuCl4]2-(aq) + 6H2O(l) LS
yellow, tetrahedral
REDOX
NOTE:
Cu (II) has configuration [Ar] 3d9 and is coloured as it is possible to promote an
electron across the split 3d orbitals by absorbing a photon.
Cu (I) has configuration [Ar]3d10 and is not coloured as due to the full 3d orbital no
electron promotion is possible.
REACTIONS OF COPPER(I)
The aqueous chemistry of copper(I) is unstable compared to copper(0) and copper (II).
high charge density of M3+ makes the solution too acidic to form the carbonate
CARBON DIOXIDE IS EVOLVED.
REACTIONS OF CHROMIUM(III)
CO32- 2 [Cr(H2O)6]3+(aq) + 3CO32-(aq) ——> 2[Cr(OH)3(H2O)3](s) + 3H2O(l) + 3CO2(g)
high charge density of M3+ makes the solution too acidic to form the carbonate
CARBON DIOXIDE IS EVOLVED.
Acidification of the yellow chromate will produce the orange dichromate(VI) ion
Reduction Chromium(III) can be reduced to the less stable chromium(II) by zinc in acid
In acidic solution, dichromate is widely used in both organic (oxidation of alcohols) and
inorganic chemistry.
It can also be used as a volumetric reagent but with special indicators as its colour
change (orange to green) makes the end point hard to observe.
• Its E° value is lower than that of Cl2 (1.36V) so can be used in the presence of Cl¯ ions
• MnO4¯ (E° = 1.52V) oxidises chloride in HCl so must be acidified with sulphuric acid
• chromium(VI) can be reduced back to chromium(III) using zinc in acid solution
CONTENTS
REACTIONS OF MANGANESE(VII)
• in its highest oxidation state therefore Mn(VII) will be an oxidising agent
• occurs in the purple, tetraoxomanganate(VII) (permanganate) ion (MnO 4¯)
• acts as an oxidising agent in acidic or alkaline solution
It must be acidified with dilute sulphuric acid as MnO 4¯ is powerful enough to oxidise
the chloride ions in hydrochloric acid.
No indicator is required; the end point being the first sign of a permanent pale pink
colour.
The carbonate is not precipitated but the hydroxide is; the high A-B
charge density of M3+ makes the solution too acidic to form a carbonate
CARBON DIOXIDE EVOLVED.
REACTIONS OF IRON(III)
Aqueous solutions contain the yellow-green, octahedral hexaaquairon(III) ion
The carbonate is not precipitated but the hydroxide is; the high A-B
charge density of M3+ makes the solution too acidic to form a carbonate
CARBON DIOXIDE EVOLVED.
The carbonate is not precipitated but the hydroxide is; the high A-B
charge density of M3+ makes the solution too acidic to form a carbonate
CARBON DIOXIDE EVOLVED.
Very sensitive; BLOOD RED COLOUR confirms Fe(III). No reaction with Fe(II)
REACTIONS OF SILVER(I)
• aqueous solutions contains the colourless, linear, diammine silver(I) ion
• formed when silver halides dissolve in ammonia
[Ag(SO3)2]3- Formed when silver salts are dissolved in sodium thiosulphate "hypo" solution.
Important in photographic fixing. Any silver bromide not exposed to light is
dissolved away leaving the black image of silver as the negative.
[Ag(CN)2]¯ Formed when silver salts are dissolved in sodium or potassium cyanide
the solution used for silver electroplating
Reduction
• Zinc metal is used to reduce transition metal ions to lower oxidation states
• It acts in acid solution as follows... Zn(s) ——> Zn2+(aq) + 2e¯
e.g. it reduces iron(III) to iron(II)
vanadium(V) to vanadium (IV) to vanadium(III)
REACTIONS OF ALUMINIUM
• aluminium is not a transition metal as it doesn’t make use of d orbitals
• BUT, due to a high charge density, aluminium ions behave as typical M 3+ ions
• aqueous solutions contain the colourless, octahedral hexaaquaaluminium(III) ion
A-B
NH3 [Al(H2O)6]3+(aq) + 3NH3(aq) ——> [Al(OH)3(H2O)3](s) + 3NH4+(aq)
white ppt. insoluble in XS NH3
AN INTRODUCTION TO
TRANSITION METAL
CHEMISTRY
The End!
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informative