The Model of Weak Covalent Bonding

Starting from this relation, Jorgensen et al. (45) proposed a model in which the antibonding energy effect JEa upon the d-like molecular orbitals is supposed to be proportional to the square of the group overlap integral Gml. This can be shown by solving the secular equation with the help of Eq. (4) and Gmi, 1. Hi, Hm (44)  

(5) therefore is valid for predominantly ionic complexes only. The metal ion orbitals usually are approximated as products of an angular and a radial function, and the Gml are expressed in terms of diatomic overlap integrals S(9jm,9 l), e.g. S(34z2,Uz) for c-overlap (og is a u-orbital of a ligand located on the z-axis) and S(3dxy,nx) for sr-overlap. 

Because e is assumed to be constant in this simple MO-model, a quantity proportional to 10 Dq can easily be calculated from diatomic overlap integrals. Fig. 2 gives the results for a hypothetical [CuFe] octahedron. The Me-F dependence of 10 Dq now has the correct sign, but the law, which sometimes has been verified by experiment (23, 58, 83), is realized only within a very small range of Me-F distances. 

In complexes of lower symmetry usually more than one linear combination of ligand orbitals wiU have non-vanishing overlap integrals with a d-orbital of given symmetry, and the quantity G ( )m. l) in Eq. (5) should be substituted by 2 Xjj). For a tetragonal complex with 

The sphtting of the octahedral eg orbitals is caused by differences of ff-antibonding energies exclusively and that of the t g orbitals by differences of a -antibonding energies only. 

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