Magnetic exchange across the cyanide bridge

The superexchange interaction in cyano-bridged transition metal dimers is analysed by a valence bond configuration interaction model in combination with fourth order perturbation theory. Ferro- and antiferromagnetic contributions to the exchange splitting are expressed in terms of metal-ligand hybridization matrix elements, metal-to-ligand, ligand-to-metal, metal-to-metal charge transfer energies, and intra-atomic exhange integrals. The formalism is simplified and we arrive at a two-parameter model, with which it is possible to estimate magnetic ordering temperatures of cubic stoichiometric Prussian Blue like phases containing first row transition metals. The relevance and applicability of our theoretical results is demonstrated on critical temperatures reported in the literature. The model accounts for both ferri- and ferromagnetic ordering temperatures. It is found that cyanide is an efficient mediator of exchange due to the participation of the pi and pi* orbitals to equal extent.