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-μ-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 et al. [Chem. Commun. <BO>1998</BO>, 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 ΔP²(d<INF>xz</INF><INF></INF>), 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.