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) Angstrom and 138.3(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 k(trans)(25 degrees C, CH3CN) = 24.6 +/- 0.6 M-1 s(-1) and k(cis)(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 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 atom transfer. These results can be explained by invoking a common intermediate, [OsIV(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)](+).