The phosphinidene complexes PhPW(CO)5 (1a) and MePW(CO)5 (1b) have been added to exocyclic olefins to yield 1-phosphaspiro[2.n]alkanes (n = 2−5). The molecular structures of the parent phosphirane 10a and the parent phosphaspiro[2.2]pentane 6a that result from addition of PhPW-(CO)5 to ethylene and methylenecyclopropane, respectively, have been determined by single-crystal X-ray analyses. The reactivity order for addition of both phosphinidene complexes to exocyclic olefins was determined as methylenecyclopentane (4) > methylenecyclobutane (3) > methylenecyclohexane (5) > methylenecyclopropane (2). PhPW(CO)5 is more selective and less reactive in its additions than MePW(CO)5. The relative rates for phosphinidene addition correlate with both 13C NMR chemical shifts of the spiro-carbon and the 1JCH coupling constants of the CH2 group of the 1-phosphaspiro[2.n] alkanes products. © 1993, American Chemical Society. All rights reserved.