Sequential photosubstitution of carbon monoxide by (E)-cyclooctene in hexacarbonyltungsten: Structural aspects, multistep photokinetics, and quantum yields

The photochemical conversion of W(CO)(6) (1) into a trans-W(CO)(4)(eta(2)-o lefin)(2) complex has been investigated using (E)-cyclooctene (eco) as a mo del olefin possessing extraordinary coordination properties. trams-W(CO)(4) (eta(2)-eco)(2) (4) is generated as an equimolar mixture of two diastereois omers (4a, S-4 symmetry; 4b, D-2 symmetry) which can be separated by fracti onal crystallization. The entire reaction sequence involves the intermediat e formation of W(CO)(5)(eta(2)-eco) (2) and cis-W(CO)(4)(eta(2)-eco)(2) (3: two diastereoisomers, 3a and 3b, with apparent C-s and C-2 symmetry, respe ctively). Complexes 2 and 3, although difficult to isolate from the photoch emical reaction mixture, are conveniently accessible via alternative therma l ligand exchange routes,The molecular structures of 2 and 4a in the crysta l were determined by X-ray diffraction techniques, The olefin double bonds, with trans-orthogonal arrangement in 4a, are eclipsed to a OC-W-CO axis in either case. The course of the conversion of 1 into the olefin-substituted products was monitored by quantitative lit spectroscopy. Photokinetic equa tions developed for this study describe the concentrations of all four comp onents as implicit; functions of the amount of light absorbed by the system , of the quantum yields of the individual photoprocesses, and of the UV-vis absorbance coefficients of the compounds involved. Based on these function al relationships, the individual quantum yields at lambda(exc) = 365 nm (Ph i(12) = 0.73, Phi(23) = 0.34, Phi(24) = 0.16, Phi(34) = 0.15) were evaluate d from a series of experimental data sets by an iterative procedure which i nvolves variation of the quantum yield input data until the best fit of the computed to the measured concentrations is achieved. Low-temperature matri x isolation techniques were employed to characterize the W(CO)(4)(eta(2)-ec o) fragment (5) as a key intermediate in the photolysis of W(CO)(5)(eta(2)- eco) (2).