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Zusammenfassung:
Continued irradiation of W(CO)6 in the presence of excess bis(trimethylsilyl)ethyne (btmse) in n-hexane solution yields W(CO)5(η2-btmse) as the sole product, with quantum yields of 0.66 and 0.69 at λexc = 365 and 313 nm, respectively. Cr(CO)6 behaves analogously, while with Mo(CO)6 the initially generated Mo(CO)5(η2-btmse) undergoes further CO substitution to form trans-Mo(CO)4(η2-btmse)2 as the second product. All four compounds were isolated in crystalline form and fully characterized by elemental analysis, MS, IR, and NMR spectroscopies as well as by single-crystal X-ray crystallography. They assume a quasi-octahedral coordination geometry with the alkyne triple bond being eclipsed to one OC−M−CO axis and, in the case of trans-Mo(CO)4(η2-btmse)2, in trans-orthogonal orientation to the second alkyne. Both Mo(CO)5(η2-btmse) and Cr(CO)5(η2-btmse) are labile toward alkyne displacement by CO, whereas W(CO)5(η2-btmse) and trans-Mo(CO)4(η2-btmse)2 undergo spontaneous 12CO/13CO exchange in the dark under mild conditions. Partially 13CO labeled samples generated in this way provide complementary CO stretching vibrational data needed for thorough analyses at the level of the energy-factored CO force field approximation. From all the structural features and spectroscopic data it is evident that bis(trimethylsilyl)ethyne acts as a two-electron donor ligand in this series of d6 carbonylmetal(0) complexes.
