Ab initio calculations of [CoY6-nXn](2+) complexes

The complete active space self-consistent field (CASSCF) and multireference second order perturbation theory (CASPT2) calculations of [CoF6](4-), [Co( H2O)(6)](2+), [Co(NH3)(6)](2+), [Co(H2O)(5)X](2+), and [Co(H2O)(4)X-2](2+) complexes (X=CH3OH, CH3SH, CH3NH2) are reported. The potential energy surfa ces of 10 lowest quartet states of [Co(H2O)(5)X](2+) complexes near the equ ilibrium geometry are calculated and splitting of triple-degenerate T-4(1g) (F), T-4(2g)(F), and T-4(1g)(P) electronic states of [Co(H2O)(6)](2+) compl ex induced by the substitution of one or two water ligands is characterized and quantified. It is shown that the energy differences between originally degenerate states are almost invariant to the changes of metal-ligand dist ances, and despite their proximity, the crossing does not occur. The coeffi cients of leading configuration of multireference wave functions of [Co(H2O )(5)X](2+) and [Co(H2O)(4)X-2](2+) complexes are shown to approach unity an d the usage of single-reference methods is justified. As a consequence, int eraction energies of the studied functional groups with Co2+ are computed a lso at the HF (Hartree-Fock), DFT (density functional theory), and MP2 (sec ond-order Moller-Plesset) levels. They are compared to CASSCF calculations and to the equivalent calculations done for Zn2+ and Ni2+ ions. The computa tional methodology for the accurate calculations of various cobalt (II) ion ic complexes is described and the implications for the theoretical investig ation of interactions of chemically and biologically important functional g roups with Co2+ are discussed. (C) 2000 American Institute of Physics. [S00 21-9606(99)30447-5].