Electronic ground state and the d-d transitions in bis(biuretato)cobaltate(III) ions

The electronic structure of a class of compounds based on the bis(biuretato)cobaltate(III) ion has been investigated by spectroscopic methods (UV-vis, CD, MCD), magnetic susceptibility, and ligand field theory. The concept of molecules in molecules has been introduced to account for the conjugated pi system of the whole ligand entity and its perturbation of the metal ion 3d orbitals. The Slater-Condon-Racah scheme was fully exploited; in particular, differences in occupation numbers of the spin orbitals have been used in the spectral assignments of the d-d transitions. The energy calculations used one sigma parameter, two pi parameters, and two Racah parameters. The pi parameters, which were derived from Orgel orbitals of chi and psi type, were found to be positive. The observed charge-transfer transitions are metal <-- ligand. The results of our calculations are in agreement with available experimental data, including the spin triplet ground state and the position of the lower d-d transitions. The approach is general and, for example, applicable to heme iron(II).