MATRIX-ISOLATION STUDIES OF ALKYNE COMPLEXES IN THE PHOTOCHEMICAL-REACTIONS OF MO(CO)(6) WITH ALKYNES

Photolysis of a low-temperature matrix X (X = Ar or CH4) doped with mo lybdenum hexacarbonyl and an alkyne RC=CR' (HC=CH, MeC=CH, or MeC=CMe) leads to the formation of the corresponding alkyne complex of molybde num pentacarbonyl, [(eta(2)-RC=CR')Mo(CO)(5)], in addition to [X... Mo (CO)(5)] and CO; the products have been detected and characterised by their IR and UV-vis absorption spectra. The species [(eta(2)-RC=CR')Mo (CO)(5)], unlike [X...Mo(CO)(5)], is stable with respect to irradiatio n in the green region of the visible spectrum, and a scheme has been d evised for the photochemical interconversion of the three species [Mo( CO)(6)], [(eta(2)-RC=CR')Mo(CO)(5)] and [X...Mo(CO)(5)] in a mixed mat rix composed of RC=CR' and X molecules. An eta(2)-alkyne complex of th e type [(RC=CH)Mo(CO)(5)] (R=H or Me) undergoes rearrangement to give what is believed to be a vinylidene complex [R(H)C=C=Mo(CO)(5)]. Conti nued irradiation at lambda = 313 nm causes the various pentacarbonyl c omplexes to lose CO with the formation of tetracarbonyl species. More significantly, irradiation at lambda = 369 nm also results in photoeje ction of CO; this favours the formation of a product believed, on the evidence of its IR spectrum and of experiments with C2D2 and (CCH2)-C- 12-C-13, to be [(eta(2)-RCCR')Mo(Co-4] (R,R' = H,H or Me,Me) in which the alkyne is acting formally as a 4e ligand. The IR spectra serve not ice of further changes on prolonged photolysis under these conditions. The results are discussed in relation to intermediates in the catalyt ic polymerization of 1-alkynes.