Eight-co-ordination. Part III. Ligand–ligand Repulsions and crystal field stabilisation energies
Relative ligand–ligand repulsion energies are calculated for square antiprismatic and dodecahedral configurations, with use of a force-law between point ligands of the type: Force ∝ r–n(n= 1, 2, 6, 7, 10, 12). The antiprismatic configuration is the more stable but the energies of the two configurations differ by not more than about 1 kcal./mole. The shielding effect of the central metal atom cannot be estimated accurately, but this effect would be appreciable only for Coulombic interactions in a highly polar complex, when the dodecahedral configuration might be stabilised by several kcal./mole. With the possible exception of the latter case, it is therefore unlikely that the configuration adopted by any eight-co-ordinate complex would be determined by inter-ligand repulsion alone.
The d-orbital energies are calculated on a point-charge model. Within the range of observed geometries the crystal-field stabilisation energies of the two configurations are similar.