SYSTEMATIC AB-INITIO STUDY OF THE LIGAND-FIELD SPECTRA OF HEXACYANOMETALATE COMPLEXES

The ground and ligand field excited states of the first-row hexacyanom etalate complexes V(CN)64-, Cr(CN)63-, Mn(CN)63-,4-, Fe(CN)63-,4-, and Co(CN)63- have been studied using the complete active space (CAS) sel f-consistent field (SCF) method and a second-order perturbation approa ch (CASPT2), which applies the CASSCF wave function as a reference. It is shown that the major features of the electronic structure of both the ground state and the excited states are properly described by a CA SSCF wave function based on an active space of the 10 orbitals directl y involved in the metal-cyanide bonding: the 2t2g and 6e(g) orbitals w ith mainly metal 3d character and their ligand counterparts 3t2g(CNpi ) and 5e(g)(CNsigma). Remaining correlation effects are dealt with in the second, CASPT2, step. Two sets of CASPT2 calculations were perform ed. In the first set, the metal 3d and C and N 2s, 2p valence electron s are correlated, while the second set also includes the metal 3s, 3p core electrons. The final CASPT2 excitation energies are in excellent agreement with the experimental excitation energies for the d3 and d6 systems. The theoretical results have also been used to judge between different experimental assignments for Mn(CN)63- and Fe(CN)63- and to predict the position of the ligand field transitions in Mn(CN)64-.