CRYSTAL-FIELD SPLITTINGS AND OPTICAL-SPECTRA OF TRANSITION-METAL MIXED-LIGAND COMPLEXES BY EFFECTIVE HAMILTONIAN METHOD

Many of the important properties of transition-metal complexes depend on the low-energy excitation spectrum formed by d-electrons of the cen tral transition-metal atom. The spectra of this type are usually fit t o the well-known crystal field theory or to the angular overlap model. The result of the fitting is a set of parameters which are considered as characteristics of the electronic structure of the complex such as strength of the ligand field or types and extent of metal-ligand bond ing. We present here a short account of the effective Hamiltonian meth od recently developed to calculate the splitting of the d-levels by th e ligands and the resulting d-d spectra of transition-metal complexes together with some results of its application to the mixed-ligand comp lexes with the general formula ML(4)Z(2), where M = V, Co, Ni; L = H2O , NH3, Py; and Z = H2O, NCS-, Cl-. Particular attention is paid to the V(H2O)(4)Cl-2 and Co(H2O)(4)Cl-2 compounds. The former seems to have tetragonal structure, whereas for the latter, our method predicts a sp atially degenerate ground state for the tetragonal arrangement of the ligands. That must lead to the Jahn-Teller distortion, which is actual ly observed. (C) 1996 John Wiley & Sons, Inc.