Ab initio calculations of the transfer parameters and coulombic repulsion and estimation of their effects on the electron delocalization and magnetic coupling in mixed-valence Keggin polyoxotungstates

In this work, we present ab initio calculations on embedded fragments that permit to extract the value of the effective electron transfer integral and coulombic repulsion between W nearest neighbour atoms in a mixed-valence alphaPW(12)O(40) Keggin polyoxoanion. This allows us to perform a quantitative study of the influence of these two parameters on the magnetic properties of Keggin polyoxoanions reduced by two electrons. We surprisingly find that the electron transfer between edge-sharing and corner-sharing WO6 octahedra have very close values, and show that the punctual charges estimation of coulombic repulsion may not be accurate enough to study the electronic distribution of the system. Finally, the parameters are introduced in a model Hamiltonian that represents the whole anion. The result is that electron transfers induce a large singlet-triplet gap in Keggin polyoxoanion reduced by two electrons, and so rationalizes its experimentally observed diamagnetism. (C) 2003 Elsevier Science Ltd. All rights reserved.