Redox chemistry of a pair of complexes that contain the bridging bis(diphenylphosphino)methane ligand and the Re26+ and Re24+ cores: Re2(.mu.-Cl)2(.mu.-dppm)2Cl4 and [Re2(.mu.-dppm)2Cl3(NCR)2]PF6 (R = CH3, C2H5, C6H5)