Symmetry-restricted covalence in ligand field theory. Part I. The relative energies of the 2B2g and 2Eg states in tetra-amminecopper(II) compounds

Abstract

The effects of symmetry-restricted covalence on the d–d spectra of tetra-amminecopper(II) compounds are discussed, with particular reference to the relative energies of the ²B_2g and ²E_g states. The d–d transition energies are expressed as the sum of a change in one-electron energy and a change in two-electron energy. The latter terms are calculated in the INDO approximation; the necessary MO mixing coefficients are obtained by appeal to experimental data and a simple approximation based on perturbation theory. For square coplanar Cu(NH₃)₄²⁺, it is shown that the ²B_2g state should lie higher in energy than the ²E_g state if, as expected in a σ-bonded system, the b_2g and e_g orbitals are accidentally degenerate. This is contrary to the predictions of simple ligand field models. If axial ligands having π-donor orbitals are allowed to approach the metal, completing the usual tetragonal octahedral co-ordination geometry, the model predicts that, notwithstanding the destabilization of the e_g orbitals as a consequence of axial π-interaction, the ²E_g state becomes higher in energy than the ²B_2g state. These results are consistent with experimental data.