Electron delocalization and magnetic state of doubly-reduced polyoxometalates

Different mechanisms of spin pairing in doubly reduced polyoxometalates are studied on the basis of quantum-chemical DFT calculations. Using the nitro syl derivative of decamolybdate [Mo10O25(OMe)(6)(NO)](-) (1) as an example, we elucidate an important role of the delocalization of "blue electrons". The charge distributions and spin states are studied for the series of isom ers of I differing by positions of methyl groups (modeled by hydrogens). Th ree different states are calculated for each isomer: spin triplet, spin-res tricted singlet, and a broken symmetry state. If the quasihomogeneous distr ibution of the "blue electrons" density is weakly perturbed by protonation, the delocalization mechanism is responsible for the spin pairing. It is ev idenced by the singlet ground state given by a spin-restricted solution. If the perturbation of charge distribution is strong enough and the "blue ele ctrons" density is localized at several metal centers, the exchange mechani sm becomes active. A lowest energy broken symmetry state indicates the anti ferromagnetic nature of the singlet ground state. The modulation of magneti c interactions in reduced polyoxoanions by external perturbations provides new possibilities for design of molecular magnetic materials.