CHARGE AND ELECTRON-TRANSFER FROM METAL-TO-CARBON BONDS OF MAIN-GROUP ORGANOMETALLICS MR(N) (M=AL, GA, ZN) TO AROMATIC N-HETEROCYCLES - COLORED PRECURSOR COMPOUNDS AND RADICAL COMPLEX-FORMATION

The coordinatively unsaturated compounds AlR(3), GaR(3) and ZnR(2), R = isopropyl, were reacted with linearly bridging pyrazine, terramethylpyrazine or 4,4'-bipyridine ligands and with potentially chelating 2,2'-bipyridine. The primary reaction products are diamagnetic dinuclear complexes ('bis-adducts') which are characterized by intense long-wavelength ligand-to-ligand charge transfer (LLCT) transitions sigma(M-R)-->pi*(L) and which may yield radical complexes after loss of R(.). The latter reaction is assumed to have a strong single electron transfer (SET) component. An isolable 'Al(II)' species (bpy)Al[CH(SiMe(3))(2)](2) has been characterized unambiguously by ESR, W-Vis and cyclic voltammetry as an Al(III) complex of the bpy radical anion. Irradiation into the fairly intense LLCT absorption bands (epsilon > 3000 M(-1) cm(-1)) of the dinuclear complexes between pyrazine and (AlPr3)-Pr-i or (GaPr3)-Pr-i resulted in bleaching and formation of a mixture of non-aromatic products. Attempts are made to correlate charge transfer properties and radical formation with the geometrical and electronic structures of the precursor complexes.