Reversible Chelate-to-Bridge Ligand Exchange in Co2(CO)4(.mu.-PhC.tplbond.CPh)(bma) and Alkyne-Diphosphine Ligand Coupling. Synthesis, Reactivity, and Molecular Structures of Co2(CO)4(.mu.-PhC.tplbond.CPh)(bma), Co2(CO)4(.mu.-PhC.tplbond.CPh){(Z)-Ph2PCH:CHPPh2}, and Co2(CO)4{.eta.2,.eta.2,.eta.1,.eta.1-(Z)-Ph2PC(Ph):(Ph)CC:C(PPh2)C(O)OC(O)}