Ab initio calculation of the magnetic exchange coupling in linear oxo-bridged heterobinuclear complexes of titanium (III), vanadium (III), and chromium (III)

Quantum chemical ab initio calculations have been performed for the magneti c exchange coupling in three linear ore-bridged heterobinuclear transition- metal complexes of the form L5M1(III)-O-M-2(III)L-5 with M-1, M-2 = Ti, V, Cr. In order to simplify the calculations, the terminal Ligands L are chose n as He-Like model Ligands with sigma-donor properties. The metal cations i n the complexes L5TiOVL5 and L5TiOCrL5 are found to be ferromagnetically co upled at the complete active space self-consistent field (CASSCF) and valen ce configuration interaction (CI) levels, with exchange integrals J of + 13 and +12 cm(-1), respectively. Inclusion of correlation effects preserves t he ferromagnetic coupling but enlarges the J values to + 31 and +24 cm(-1). The L5VOCrL5 complex, on the other hand, is antiferromagnetic with I = -46 cm(-1) at the valence CI level and -106 cm(-1) after inclusion of electron correlation. A detailed analysis is performed in order to decompose J into a "potential exchange" and a "superexchange" contribution and to rationali ze the different magnetic behavior of the three complexes. The strong influ ence of electron correlation and relaxation effects on the numerical values of I is analyzed as well. (C) 2000 John Wiley & Sons, Inc.