The series of phenoxide complexes cis-L 4 Os(H)OC 6 H 4 Z (L ) PMe 3 ; Z ) H, OMe, CF 3 , NH 2 , CN), anilides cis-L 4 Os(H)NHC 6 H 4 Z (Z ) H, OMe, CF 3 ), and thiophenoxides cis-L 4 Os(H)-SC 6 H 4 Z (Z ) H, OMe) have been prepared by treatment of fac-L 3 Os(H)(η 2 -CH 2 PMe 2 ) with the corresponding neutral arene. 1 H NMR spectra of coordinated phenoxides and thiophenoxides show rapid phenyl group rotation, while that of the anilides is slow. Rates and stereochemistry of substitution of P(CH 3 ) 3 (L) by P(CD 3 ) 3 (L′) were determined by 31 P NMR in benzene at 80°C. Anilides substitute first only in the mutually trans sites (a sites) with cleanly first-order kinetics and subsequently into the site trans to the anilide (site c). The latter shows non-first-order behavior that is accurately modeled by iterative kinetics calculations using a mechanism of L dissociation only from the a sites presumably to give a quasi-trigonal-bipyramidal intermediate stabilized by π-electron donation from the anilide lone pair. A three-point Hammett plot against σ p yielding F ) -1.8 is consistent with transition state stabilization by π-donation. Association of L′ occurs only into site a, but subsequent substitution allows the resident L′ to move into site c. Thiophenoxides exhibit substitution rates and stereochemical patterns very similar to those of the anilides and are believed to proceed by the same mechanism. Phenoxide complexes incorporate L′ into all sites, with each site incorporating phosphine independently of the others since exchange rates at all sites are first order. A Hammett plot of exchange rates against sigma minus (σ -) is somewhat scattered (R 2 ) 0.96) but exhibits a positive slope F -) +0.36. Phenoxide dissociation is postulated, but the fact that substantial concentrations of intermediates partially substituted in all positions is seen during the reaction is inconsistent with ratedetermining phenoxide dissociation. An ionization preequilibrium followed by slower phosphine exchange steps in the ion pair is postulated. Treatment of L 4 Os(H)(OC 6 H 4 CN) with excess L in propylene carbonate, DMSO-d 6 , DMF, or CD 3 NO 2 at 80°C all resulted in conversions to [L 5 OsH][OAr]. These results suggest that low steady-state concentrations of [L 5 OsH][OAr] ion pairs in benzene are possible, consistent with an ion pair mechanism for ligand exchange.