CHEMICAL AND ELECTROCHEMICAL GENERATION OF HYDRIDE-FORMING CATALYTIC INTERMEDIATES (BPY)M(C(N)R(N)), M=RH, IR(N=5), M=RU, OS (N=6) - COORDINATIVELY UNSATURATED GROUND-STATE MODELS OF MLCT EXCITED-STATES

The d(6) metal complex ions [(bpy)MCl(C(n)R(n))](+) [bpy 2,2'-bipyridine; M = Rh, Ir (n = 5, R = Me); M = Ru (n = 6; C(6)R(6) C6H6, p-MeC(6)H(4)(i)Pr, C(6)Me(6)); M = Os (n 6; C(6)R(6) p-Me(6)H(4)(i)Pr, C(6)Me(6))] were prepared as hexafluorophosphate salts and studied by cyclic voltammetry. In each case, a two-electron reduction produced highly colored coordinatively unsaturated species (bpy)M(C(n)R(n)) which could also be obtained via chemical reduction. H-1 and C-13 NMR high field shifts of bpy resonances, cathodically shifted secondary one-electron reduction waves, and the vibrationally structured long-wavelength absorption features in the visible region of these neutral compounds all suggest an oxidation state distribution involving a partially reduced bpy ligand. Such an electronic structure would correspond to the typical formulation of low-lying MLCT excited states in conventional d(8) metal complexes and may be relevant for the functioning of the systems as catalysts for hydride transfer, including H-2 evolution. Stable hydride intermediates of this catalysis were isolated and characterized in the case of the 5d systems [(bpy)MH(C(n)R(n))](+) [M = Ir (n 5) and Os (n = 6)] which could be reversibly reduced to neutral radical complexes as evident from a resolved EPR spectrum.