Continued irradiation of W(CO)(6) in the presence of excess bis(trimethylsilyl)ethyne (btmse) in n-hexane solution yields W(CO)(5)(eta(2)-btmse) as the sole product, with quantum yields of 0.66 and 0.69 at lambda(exc) = 365 and 313 nm, respectively. Cr(CO)(6) behaves analogously, while with MO(CO)(6) the initially generated Mo(CO)(5)(eta(2)-btmse) undergoes further CO substitution to form trans-Mo(CO)(4)(eta(2)-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)(eta(2)-btmse)(2), in trans-orthogonal orientation to the second alkyne. Both Mo(CO)(5)(eta(2)-btmse) and Cr(CO)(5)(eta(2)-btmse) are labile toward alkyne displacement by CO, whereas W(CO)(5)(eta(2)-btmse) and trans-MO(CO)(4)(eta(2)-btmse)(2) undergo spontaneous (CO)-C-12/(CO)-C-13 exchange in the dark under mild conditions. Partially (CO)-C-13 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 d(6) carbonylmetal(0) complexes.