Reactions of co-ordinated ligands. Part 54. Synthesis of (1,2,3-η)-trans-butadienyl complexes by deprotonation or desilylation of cationic molybdenum 1,3-diene complexes; formation of η4-vinylketene complexes and crystal structures of [Mo{(1,2,3-η)-trans-CH2CHCCH2}(CO)2(η-C5Me5)], [Mo{η3-CH2CC(Me)CH2CO}(CO)2(η-C5Me5)] and [Mol{η4-CH2CHC(Me)CO}(CO)(η-C5Me5)]