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 magnetic exchange coupling in three linear oxo-bridged heterobinuclear transition-metal complexes of the form L5M1(III)OM2(III)L5 with M1, M2=Ti, V, Cr. In order to simplify the calculations, the terminal ligands L are chosen as He-like model ligands with σ-donor properties. The metal cations in the complexes L5TiOVL5 and L5TiOCrL5 are found to be ferromagnetically coupled at the complete active space self-consistent field (CASSCF) and valence configuration interaction (CI) levels, with exchange integrals J of+13 and +12 cm−1, respectively. Inclusion of correlation effects preserves the ferromagnetic coupling but enlarges the J values to +31 and +24 cm−1. The L5VOCrL5 complex, on the other hand, is antiferromagnetic with J=−46 cm−1at the valence CI level and −106 cm−1after inclusion of electron correlation. A detailed analysis is performed in order to decompose J into a “potential exchange” and a “superexchange” contribution and to rationalize the different magnetic behavior of the three complexes. The strong influence of electron correlation and relaxation effects on the numerical values of J is analyzed as well.