CHM 501 Lecture

Valence Bond Theory

Effective Atomic Number Rule (EAN): complexes with 18 (sometimes 16) electrons are stable (analogous to octet rule for main group elements)

Count the metal valence electrons and the electrons donated by the ligands; if this adds up to 18 (or 16), then the complex is predicted to be stable

EAN is most effective for low oxidation states and organometallic complexes

Organometallic complex: a compound with a metal to carbon bond.

Unlike the main group, d orbital lone pairs are generally not stereochemically active so the geometries of these compounds normally are determined solely by the number of ligands.



Fe d8 (Note: when in a bonding situation, all of the electrons on a metal move to the orbital with the lowest n quantum number, similar to when ionization happens.)

CO 5(2) = 10

8 + 10 = 18


Co d6

:NH3 6(2) = 12

6 + 12 = 18


Fe2+ d6

cp 2(6) = 12

6 + 12 = 18


Co2+ d7

cp 2(6) = 12

7 + 12 = 19 not stable but Co(cp)2+ is stable


on each Mn: Mn d7

CO 5(2) = 10

M-M bond, 1

7 + 10 + 1 = 18


Pt2+ d8

Cl 3(2) = 6

C2H4, 2

8 + 6 + 2 = 16

Molecular orbital approach (Ligand Field Theory)

A Molecular Orbital Diagram using only sigma bonding can be constructed for a general Oh complex:

Bond Order = 6 - n(eg)/2

The sigma bonds are largely ligand based orbitals but take the appearance of d2sp3 hybrid orbitals about the metal.

The nb and lowest energy σ* orbitals are similar to the Crystal Field Theory description. The nb orbitals are strictly d like (dxy, dyz, dxz) and the σ* are mostly d orbital like (dx2–y2, dz2).

Inclusion of π bonding means using the t2g orbitals for overlap:

If the t2g orbitals are empty, then the ligand needs filled π orbitals to create a M-L π bond; the t2g orbitals become somewhat antibonding, are destabilized, and 10Dq is reduced.

If the t2g orbitals are occupied, then the ligand needs unfilled π* orbitals to create the M-L π bond; the t2g orbitals become somewhat bonding, are stabilized, and 10 Dq is increased.

This is known as π backbonding. It is the reason CO is such a strong ligand.