Ab initio (density functional) study of the exchange coupling constant in di-μ-oxo-bridged copper(II) dimers:: A valence bond/broken symmetry approach

Exchange coupling constants for di-mu-oxo-bridged copper(II) dimers with varying Cu-O-Cu bond angles have been calculated by means of density functional, broken symmetry (BS) techniques. The surprinsingly large ferromagnetism recently computed by Ruiz ct al, [Chem. Commun. 1998, 2767] for such model complexes has been rationalized within the framework of Kahn's valence bond model of molecular magnetism, although with a new twist. In effect, by defining and using the quantity Delta P-2(d(xz)), the difference of squared copper triplet and BS spin populations, we show that Kahn's (supposedly) antiferromagnetic term can turn out ferromagnetic, as exemplified in a spectacular way for the title compound.