Mechanistic control of product selectivity. Reactions between cis-/trans-[Os-VI(tpy)(Cl)(2)(N)](+) and triphenylphosphine sulfide

Reactions between the Os(VI)-nitrido complexes cis- and trans-[Os-VI(tpy)(C l)(2)(N)](+) (tpy is 2,2':6',2"-terpyridine) and triphenylphosphine sulfide , SPPh3, give the corresponding Os(IV)-phosphoraniminato, [Os-IV(tpy)(Cl)(2 )(NPPh3)](+), and Os(II)-thionitrosyl, [Os-II(tpy)(Cl)(2)(NS)](+), complexe s as products. The Os-N bond length and Os-N-P angle in cis-[Os-IV(tpy)(Cl) (2)(NPPh3)](PF6) are 2.077(6) Angstrom and 138.3(4)degrees. The rate law fo r formation of cis- and trans-[Os-IV(tpy)(Cl)(2)(NPPh3)](+) is first order in both [Os-VI(tpy)(Cl)(2)(N)](+) and SPPh3 with k(trans)(25 degrees C, CH3 CN) = 24.6 +/- 0.6 M-1 s(-1) and k(cis)(25 degrees C, CH3CN) = 0.84 +/- 0.0 9 M-1 s(-1). As found earlier for [Os-II- (tpm)(Cl)(2)(NS)](+), both cis- a nd trans-[Os-II(tpy)(Cl)(2)(NS)](+) react with PPh3 to give [Os-IV(tpy)(Cl) (2)(NPPh3)](+) and SPPh3, For both complexes, the reaction is first order i n each reagent with k(trans)(25 degrees C, CH3CN) = (6.79 +/- 0.08) x 10(2) M-1 s(-1) and k(cis)(25 degrees C, CH3CN) = (2.30 +/- 0.07) x 10(2) M-1 s( -1). The fact that both reactions occur rules out mechanisms involving S at om transfer. These results can be explained by invoking a common intermedia te, [OsIV(tpy)(Cl)(2)(NSPPh3)](+), which undergoes further reaction with PP h3 to give [Os-IV(tpy)(Cl)(2)(NPPh3)](+) and SPPh3 or with [Os-VI(tpy)(Cl)( 2)(N)](+) to give [Os-IV(tpy)(Cl)(2)(NPPh3)](+) and [Os-II(tpy)(Cl)(2)(NS)] (+).