Ab initio study of the singlet-triplet splitting in reduced polyoxometalates

A valence bond analysis of the wave function of doubly reduced polyoxometales is presented, using the M(6)O(19) Lindqvist structure as test case. By a unitary transformation of the delocalised valence orbitals to localised metal centred orbitals, the multiconfigurational wave function is mapped onto a valence bond function with three different types of configurations: the two electrons are on the same site, on neighbouring sites, or on next-nearest neighbour sites. The inspection of the relative weights of these configurations for triplet and singlet state shows that the triplet-coupled electrons are confined to a smaller volume, and hence have a higher energy than the singlet-coupled electrons. This is in line with the experimental observation that the doubly reduced polyoxometalates show non-mangetic behaviour.