TRISUBSTITUTED HETEROPOLYTUNGSTATES AS SOLUBLE METAL-OXIDE ANALOGS - ISOLATION AND CHARACTERIZATION OF [(C(5)ME(5))RH-CENTER-DOT-P2W15NB3O62](7-) AND [(C6H6)RU-CENTER-DOT-P2W15NB3O62](7-), INCLUDING THE FIRST CRYSTAL-STRUCTURE OF A DAWSON-TYPE POLYOXOANION-SUPPORTED ORGANOMETALLIC COMPLEX

The heteropolyoxoanion-supported complexes [(C(5)Me(5))Rh.P2W15Nb3O62] (7-), 1, and [(C6H6)Ru.P2W15Nb3O62](7-), have been prepared by additio n of [(C(5)Me(5))Rh(CH3CN)(3)](2+) or [(C6H6)Ru(CH3CN)(3)](2+) to a so lution of P2W15Nb3O629- in acetonitrile. Isolation of these complexes as homogeneous, yellow solids as their all-[(n-C4H9)(4)N](+) salts (fo r 1) or as complexes with mixed [(n-C4H9)4N](+)/Na+ cation composition (for 1 and 2) was accomplished by repeated reprecipitation from aceto nitrile with ethyl acetate. Molecular formulas for these complexes wer e established by complete elemental analyses, in conjunction with a se dimentation-equilibrium molecular-weight measurement. Further characte rization in solution relied heavily on multinuclear NMR spectroscopy. The solution data are in accord with [(C(5)Me(5))Rh](2+) and [(C6H6)Ru ](2+) being supported on three niobium-bridging oxygens on the ''Nb3O9 3-'' surface Of the heteropolyoxoanion. The structural characterizatio n of 1 in the solid state was accomplished by a single-crystal X-ray s tructural analysis: P6(3)/m; a = 20.544(5), b = 20.544(5), c = 34.648( 6) Angstrom; Z = 2; R = 0.096 for 3023 observed independent reflection s. The Rh atom in 1 lies on the polyoxoanion's 3-fold axis within expe rimental error and is bonded to the three Nb-O-Nb bridging oxygen atom s (Rh-ONb2 2.06 Angstrom) that cap the P2W15Nb3O629- Dawson-type polyo xoanion. This crystallographic analysis, although limited by considera ble disorder, represents the first solid-state structure of a Dawson-t ype heteropolyoxoanion-supported organometallic complex.