Synthesis, characterization and reactivity of platinum-substituted ketenes [PtX{eta(1)-C(PPh3))CO}L-2]BF4, (X = Me, Cl; L-2=1,5-cyclooctadiene, 1,2-bis(diphenylphosphino)ethane, cis-1.2-bis(diphenylphosphino)ethene). Steric and electronic effects of the ancillary ligands