NMR studies of the structure, isomerism and photochemical reactions of trans-[W(CO)4(η2-alkene)2] complexes

UV broad-band photolysis of W(CO)6 in the presence of propene and 1-butene has been used to synthesise the bis-alkene complexes trans-[W(CO)4(η2-CH2CHCH3)2] 1 and trans-[W(CO)4(η2-CH2CHCH2CH3)2] 2. The 1H- and 13C-NMR spectra are discussed in detail in relation to the structure and isomerism of these alkene compounds. On the evidence of the spectra measured at different temperatures, the barrier to alkene rotation is ca. 46 and 48 kJ mol−1 in 1 and 2, respectively. The NMR method has been used for the identification and characterisation of the unstable alkene=propene or 1-butene complexes [W(CO)5(η2-alkene)] and cis-[W(CO)4(η2-alkene)2] and of hydrido derivatives formed in photochemical reactions of trans-[W(CO)4(η2-alkene)2] occurring in solution at ca. 200 K.