AN AB-INITIO STUDY OF THE GEOMETRY DEPENDENCE OF THE MAGNETIC EXCHANGE COUPLING IN OXO-BRIDGED BINUCLEAR CHROMIUM(III) COMPLEXES

Quantum-chemical ab initio calculations have been performed for the geometry dependence of the energies of the low-lying electronic states and the superexchange coupling constant J in mono-ore-bridged binuclear Cr(III) complexes of the form L(5)Cr(III)OCr(III)L(5). The terminal ligands have been replaced by model ligands, the calculations have been carried out at the CASSCF, full valence CI, and MC-CEPA levels. The following geometrical parameters have been varied: Cr-O distance r, Cr-O-Cr bending angle phi, and the angle alpha describing the rotation of one L(5)Cr unit against the other through the CrOCr axis in the linear L(5)CrOCrL(5) complex. It is found that J decays approximately exponentially with increasing r, decreases rapidly if the CrOCr bridge deviates from linearity, and is virtually independent of alpha. A rationalization of these results is given as well as a comparison with experimental data.