Binuclear rhenium complexes bridged by phosphine bidentate ligands, Re2(CO)6(.mu.-Ph2PCH2PPh2)2, Re2(CO)6(.mu.-Ph2PCH2PPh2)(.mu.-Me2PCH2PMe2), and Re2(CO)6(.mu.-Me2PCH2PMe2)2. Preparation and reactions with alkynes, water, methanol, and a hydrogen donor