Mechanistic Control of Product Selectivity. Reactions between cis-/trans-[OsVI(tpy)(Cl)2(N)]+ and Triphenylphosphine Sulfide

Reactions between the Os(VI)-nitrido complexes cis- and trans-[Os(VI)(tpy)(Cl)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)]+, complexes 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) A and 138.4(4) degrees. The rate law for 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 ktrans(25 degrees C, CH3CN) = 24.6 +/- 0.6 M(-1) s(-1) and kcis(25 degrees C, CH3CN) = 0.84 +/- 0.09 M(-1) s(-1). As found earlier for [Os(II)(tpm)(Cl)2(NS)]+, both cis- and 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 in each reagent with ktrans(25 degrees C, CH3CN) = (6.79 +/- 0.08) x 10(2) M(-1) s(-1) and kcis(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 atom transfer. These results can be explained by invoking a common intermediate, [Os(IV)(tpy)(Cl)2(NSPPh3)]+, which undergoes further reaction with PPh3 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)]+.