The pentacarbonyl(? 2 -bis(trimethylsilyl)ethyne)tungsten(0), W(CO) 5 (? 2 -btmse), reacts with tricyclohexylphosphine, PCy 3 , 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 (PCy 3 ) and trans-W(CO) 4 (PCy 3 ) 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)(PCy 3 ), with a second PCy 3 molecule. The intermediate cis-W(CO) 4 (? 2 -btmse)(PCy 3 ) complex could not be detected even in the solution. The cis-W(CO) 4 (PCy 3 ) 2 complex was observable, however, found to be instable and rapidly isomerizes to trans-W(CO) 4 (PCy 3 ) 2 . The crystal and molecular structure of W(CO) 5 (PCy 3 ) was determined and compared with those of trans-W(CO) 4 (PCy 3 ) 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) A. The W-P distance is 2.5794(12) A. Maximum deviation from an ideal octahedral coordination angle is observed to be 95.68(17) o . All three cyclohexyl rings are in chair configuration.