Phosphine ligand exchange in tetrakis(trimethylphosphine)(hydrido)osmium anilides, phenoxides, and thiophenoxides. Examples of anion dissociation andof labilization by ligand pi-base effects

The series of phenoxide complexes cis-L4Os(H)OC(6)H(4)Z (L = PMe3; Z = H, O Me, CF3, NH2, CN), anilides cis-L4Os(H)NHC(6)H(4)Z (Z = H, OMe, CF3), and t hiophenoxides cis-L4Os(H)SC(6)H(4)Z (Z = H, OMe) have been prepared by trea tment of fac-L3Os(H)(eta(2)-CH2PMe2) with the corresponding neutral arene. IH NMR spectra of coordinated phenoxides and thiophenoxides show rapid phen yl group rotation, while that of the anilides is slow. Rates and stereochem istry of substitution of P(CH3)(3) (L) by P(CD3)(3) (L') were determined by P-31 NMR in benzene at 80 degrees C. Anilides substitute first only in the mutually trans sites (a sites) with cleanly first-order kinetics and subse quently into the site trans to the anilide (site c). The latter shows non-f irst-order behavior that is accurately modeled by iterative kinetics calcul ations using a mechanism of L dissociation only from the a sites presumably to give a quasi-trigonal-bipyramidal intermediate stabilized by pi-electro n donation from the anilide lone pair. A three-point Hammett plot against s igma(p) yielding rho = -1,8 is consistent with transition state stabilizati on by pi-donation. Association of L' occurs only into site a, but subsequen t substitution allows the resident L' to move into site c. Thiophenoxides e xhibit substitution rates and stereochemical patterns very similar to those of the anilides and are believed to proceed by the same mechanism. Phenoxi de complexes incorporate L' into all sites, with each site incorporating ph osphine independently of the others since exchange rates at all sites are f irst order. A Hammett plot of exchange rates against sigma minus (sigma(-)) is somewhat scattered (R-2 = 0.96) but exhibits a positive slope rho(-) = +0.36, Phenoxide dissociation is postulated, but the Tact that substantial concentrations of intermediates partially substituted in all positions is s een during the reaction is inconsistent with rate-determining phenoxide dis sociation. An ionization preequilibrium followed by slower phosphine exchan ge steps in the ion pair is postulated. Treat;ment of L4Os(H)(OC6N4CN) with excess L in propylene carbonate, DMSO-d(6), DMF, or CD3NO2 at 80 degrees C all resulted in conversions to [L5OsH][OAr]. These results suggest that lo w steady-state concentrations;ions of [L5OsH][OAr] ion pairs in benzene are possible, consistent with an ion pair mechanism for ligand exchange.