On the Question of Mixed-Valent States in Ligand-Bridged Dinuclear Organoplatinum Compounds RkPt(m-L)PtRkn, k = 2 or 4. Organometallics, (17)16:3532--3538, 1998. [PUMA: bridged cyclic dinuclear ligand organo organoplatinum organoplatinum;mixed organoplatinum;platinum prepn;spectroelectrochem pyrazine tetrazine valent voltammetry]