MIXED-VALENCE POLYOXOMETALATE CLUSTERS .1. DELOCALIZATION OF ELECTRONIC PAIRS IN DODECANUCLEAR HETEROPOLY BLUES WITH KEGGIN STRUCTURE

The problem of delocalization of a pair of electrons over dodecanuclea r polyoxometalate clusters with the Keggin structure is considered wit h the aim of explaining the spin pairing in these multi-nuclear mixed- valence systems. A general approach that considers the Coulomb interac tions between the two delocalized electrons, as well as the single and double electron transfer processes which can be operative in delocali zation of the electronic pairs is developed. The new approach is based on the site-symmetry concept which makes possible a group theoretical classification for the delocalized states of electronic pairs. This p rocedure proves to be very efficient in the calculation of the transfe r matrices which are expressed in terms of the Coulomb energy, and the single- and double-transfer parameters. The influence of these electr onic parameters on the spectrum of the low-lying energy levels of the cluster is discussed, and the conditions giving rise to the stabilizat ion of a singlet ground spin state for the electronic pair are elucida ted.