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 central transition-metal atom. The spectra of this type are usually fit to 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 bonding. We present here a short account of the effective Hamiltonian method recently developed to calculate the splitting of the d-levels by the ligands and the resulting d-d spectra of transition-metal complexes together with some results of its application to the mixed-ligand complexes with the general formula ML₄Z₂, where M = V, Co, Ni; L = H₂O, NH₃, Py; and Z = H₂O, NCS^-, Cl^-. Particular attention is paid to the V(H₂O)₄Cl₂ and Co(H₂O)₄Cl₂ compounds. The former seems to have tetragonal structure, whereas for the latter, our method predicts a spatially degenerate ground state for the tetragonal arrangement of the ligands. That must lead to the Jahn-Teller distortion, which is actually observed.