Molecular orbital approach (Ligand Field Theory)

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

Bond Order = 6 - n(e_g)/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 (d_xy, d_yz, d_xz) and the σ* are mostly d orbital like (d_x2_–y2, d_z2).

Inclusion of π bonding means using the t_2g orbitals for overlap:

If the t_2g orbitals are empty, then the ligand needs filled π orbitals to create a M-L π bond; the t_2g orbitals become somewhat antibonding, are destabilized, and 10Dq is reduced.

If the t_2g orbitals are occupied, then the ligand needs unfilled π* orbitals to create the M-L π bond; the t_2g 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.