Investigation into the reactivity of the metal-metal triple bond in [Cp'(CO)2M]2 (Cp' = .eta.5-C5Me5', M = Mo, W) vs. elemental sulfur. Formation of different Cp'2M2S4 isomers and crystal structures of Cp'2Mo2(.mu.-S2)(.mu.-S)2 and Cp'2(CO)2W2(.mu.-S)2S