The pentacarbonyl(?(2)-bis(trimethylsilyl)ethyne)tungsten(0), W(CO)(5)(?(2)-btmse), reacts with tricyclohexylphosphine, PCy3, to yield two stable endproducts which could be isolated and fully characterized by using the single crystal X-ray diffractometry and the MS, IR, and NMR spectroscopy: W(CO)(5)(PCy3) and trans-W(CO)(4)(PCy3)(2). The former complex is the alkyne substitution product, while the latter one is formed from the conversion of its labile cis-isomer, which is generated by further reaction of the CO substitution product, cis-W(CO)(4)(?(2)-btmse)(PCy3), with a second PCy3 molecule. The intermediate cis-W(CO)(4)(?(2)-btmse)(PCy3) complex could not be detected even in the solution. The cis-W(CO)(4)(PCy3)(2) complex was observable, however, found to be instable and rapidly isomerizes to trans-W(CO)(4)(PCy3)(2). The crystal and molecular structure of W(CO)(5)(PCy3) was determined and compared with those of trans-W(CO)(4)(PCy3)(2). The coordination sphere around the W atom is a slightly distorted octahedron, involving five carbonyls and one phosphine. The W-C distances have values between 1.986(6) and 2.042(6) Angstrom. The W-P distance is 2.5794(12) Angstrom. Maximum deviation from an ideal octahedral coordination angle is observed to be 95.68(17)degrees. All three cyclohexyl rings are in chair configuration. (C) 2003 Elsevier B.V. All rights reserved.