IC - Lecture 6A
IC - Lecture 6A
IC - Lecture 6A
Sabuj Kundu
sabuj@iitk.ac.in
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18-Electron Rule
❑The 18e rule is a way to help us decide whether a given
d-block transition metal organometallic complex is likely
to be stable.
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Steps for ionic counting method:
❑ Determine the overall charge of the metal complex.
❑ Add up the electron count of the metal center and the ligands.
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Common Ligands and Their Electron Counts
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Ligand Ionic Charge
H 2 (H-) -1
Cl, Br, I 2 (X-) -1
OH, OR 2 (OH-,OR-) -1
CN 2 (CN-) -1
❑The two resonance forms show how we can consider allyl groups
in which all three carbons are bound to the metal as LX ligands.
❑In such a case, the hapticity of the ligand, the number of ligand
atoms bound to the metal, is three and so A, referred to as bis-η3-
allyl nickel, or [Ni(η3-C3H5)2].
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A
Common Ligands and Their Electron Counts
❑The Greek letter κ (kappa) is normally used instead of η when
describing ligands that bind via heteroatoms, such as κ2-acetate.
❑The allyl group can also bind in another way (B). Since only one
carbon is now bound to the metal, this is the η1-allyl form.
❑In this bonding mode, the allyl behaves as an X-type ligand, like a
methyl group, and is therefore a 1e ligand on the covalent model and
a 2e ligand on the ionic model. 10
hx Hapticity
❑ “eta-x” indicate how many contiguous donor atoms of a p-system
were coordinated to a metal center. Hapticity is used to describe
the bonding mode of a ligand to a metal center.
❑For all-carbon based ligands:
❑x = odd number; indicate anionic ligands (e.g., h5-Cp, h1-allyl or h3-
allyl). The no of electrons donated (ionic method) by the ligand is
equal to x + 1.
❑x= even number; indicate neutral carbon p-system ligands (e.g., h6-
C6H6, h2-CH2=CH2, h4-butadiene). The no of electrons donated by
the ligand equal to x.
Varying Hapticity: Varying Electron Contribution
2 electron donor
4 electron donor
6 electron donor
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Electron counting on the ionic models
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Electron counting on the ionic models
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Ionic method Electron Count
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Ionic method Electron Count
Rh(+1) d8
PR3 2e-
h4-C5H5Me 4e-
h3-C3H5- 4e-
Total: 18e-
Re(+1) d6
2 PR3 4e-
2 CO 4e-
CH3- 2e-
CH2=CH2 2e-
Total: 18e-
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Bridging Ligands: Electron Counts
❑Bridging ligands are very common and are prefixed by the symbol μ.
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18-electron Rule: Limitations
❑There are many cases in which the electron count for a stable
complex is not 18; examples are
❑MeTiCl3,8e;
❑Me2NbCl3, 10e;
❑WMe6, 12e;
❑Pt(PCy3)2, 14e;
❑[M(H2O)6]2+ (M = V, 15e; Cr, 16e; Mn, 17e; Fe, 18e),
❑CoCp2, 19e;
❑ NiCp2, 20e;
❑For the 18e rule to be useful, we need to be able to predict when it
will be obeyed and when it will not.
❑The rule works best for hydrides and carbonyls because these are
sterically small, high-field ligands. Because they are small, as many
generally bind as are required to achieve 18e.
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18-electron Rule: Limitations
❑The rule works least well for high-valent metals with weak-field
ligands. In the hexaaqua ions [M(H2O)6]2+ (M = V, Cr, Mn, Fe, Co, Ni),
the structure is the same whatever the electron count of the metal
and so must be dictated by the fact that six H2O’s fit well around a
metal ion.
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Mo(IV): 2
2 h6 Cp: 6X2= 12
Ph: 2
H: 2
Total: 18
Rh(I): 8
h6 Cp: 6
h4 Diene: 4
Total: 18
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W(II): 4
h6 C6H6: 6
Cl: 2
h3 allyl: 4
PPh3: 2
Total: 18
Fe(II): 6
h5 alkene: 6
3 CO: 6
Total: 18
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Mo(II): 4
h6 C6H6: 6
h3 alkene: 4
Cl: 2
Bridging Cl: 2
Total: 18
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