Photochemical synthesis, spectroscopic properties and barriers to alkene rotation in the novel bis(alkene) tetracarbonyl complexes of tungsten

Photolysis of W(CO)(6) in the presence of twentyfold alkene excess (1-pentene, 1-hexene, 2-hexene, 3-hexene, 1-heptene, 1-octene, 1-decene, cyclopentene, cyclohexene cycloheptene and cyclooctene) in n-hexane leads to the formation of the corresponding bis(alkene)tetracarbonyl complexes of tungsten via the less stable [(eta(2)-alkene)W(CO)(5)] complexes; the products have been isolated and characterized by their IR, UV-visible and NMR spectra. Bis(alkene)tetracarbonyl complexes of tungsten exhibit fluxional behaviour on the NMR time scale due to rotation of the alkene ligands around the axis defined by the metal and the midpoint of the C-2 linkage. The barriers to alkene rotation, which reflects the energy difference between the ortogonal and the parallel arrangement of the two C=C units, of a number of bis(alkene) complexes, have been determined using variable-temperature C-13 NMR spectroscopy.