Phosphinidene complex Ph-P-W(CO)(5) (2), generated from the appropriate phosphanorbornadiene complex 1, reacts with olefins to give the expected phosphiranes. The relative olefin reactivity toward 2 in toluene at 55 degrees C is tetramethylethylene (0.2), trimethylethylene (0.8), isobutylene (2-4), cis-2-butene (1.0), trans-2-butene (0.3), and 1-butene (0.2). The modest stereoselectivity of the phosphinidene addition reactions is reflected in the syn-/anti-phosphirane product ratios of several olefins and results form opposing steric and electronic influences. For cis-2-butene the syn/anti ratio is 0.25 and the X-ray structure of the anti-isomer was determined. The surprising lack of olefin selectivity is compared with the olefin reactivities of various carbenes and particularly that of isopropylidene. The phosphinidene-olefin additions behave analogously to the carbene cyclopropanations and suggest the importance of entropic factors in the thermodynamically controlled reactions.