Origin of the Paramagnetic Properties of the Mixed-Valence Polyoxometalate [GeV14O40]8- Reduced by Two Electrons: Wave Function Theory and Model Hamiltonian Calculations

The aim of the work is to give an explanation of the magnetic properties of a mixed-valence [GeV14O40](8-) polyoxometalate reduced by two electrons, which, in contrast to what happens in other two-electron-reduced polyoxometalates, does not show any magnetic coupling between the two unpaired electrons. For this purpose, a quantitative evaluation of the microscopic electronic parameters (electron transfer, magnetic coupling, magnetic orbital energy, and Coulomb repulsion) of the mixed-valence polyoxometalate cluster is performed. The parameters are extracted from valence-spectroscopy large configuration interaction (CI) calculations on embedded fragments. Then, these parameters are used in an extended t-J model Hamiltonian suited to model the properties of the whole anion. The analysis of the wave functions of the lowest singlet and triplet states and of the microscopic parameters emphasizes that the electron delocalization in this mixed-valence cluster is such that each impaired electron is almost trapped in a different half of the polyoxovanadate, thus disabling any exchange interaction between them. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)