Oxidative Addition of Diorganyl Ditellurides to [Mn(CO)5]-: Crystal Structures of cis-[Na-18-crown-6-ether.cntdot.2THF][Mn(CO)4(TePh)2], [PPN][(CO)3Mn(.mu.-TePh)3Mn(CO)3], (CO)3Mn(.mu.-TePh)3Co(CO)(.mu.-TePh)2Mn(CO)4, and (CO)4Mn(.mu.-TePh)2Mn(CO)4