Reaction between the Os(VI)-hydrazido complex, trans-[Os-VI(tPY)(CI)(2)(NN(CH2)(4)O)](2+) (tpy = 2,2 ' :6 ' ,2 " -terpyridine and O(CH2)(4)N- = morpholide), and a series of N-or O-bases gives as products the substituted Os(VI)-hydrazido complexes, trans-[Os-VI(4 ' -RNtpy)(Cl)(2)(NN(CH2)(4)O)](2+) or trans- [Os-VI(4 ' -ROtpy)(CI)(2)(NN(CH2)(4)O)](2+) (RN- = anilide (PhNH-); S,S-diphenyl sulfilimide (Ph2S=N-); benzophenone imide (Ph2C=N-); piperidide ((CH2)(5)N-); morpholide (O(CH2)(4)N-); ethylamide (EtNH-); diethylamide (Et2N-); and tert-butylamide (t-BuNH-) and RO- = tert-butoxide (t-BuO-) and acetate (MeCO2-). The rate law for the formation of the morpholide-substituted complex is first order in trans-[Os-VI(tpy)(Cl)(2)(NN(CH2)(4)O)](2+) and second order in morpholine with k(morp)(25 degreesC, CH3CN) = (2.15 +/- 0.04) x 10(6) M-2 s(-1). Possible mechanisms are proposed for substitution at the 4 ' -position of the tpy ligand by the added nucleophiles. The key features of the suggested mechanisms are the extraordinary electron withdrawing effect of Os(VI) on tpy and the ability of the metal to undergo intramolecular Os(VI) to Os(IV) electron transfer. These substituted Os(Vl)-hydrazido complexes can be electrochemically reduced to the corresponding Os(V), Os(IV), and Os(III) forms. The Os-N bond length of 1.778(4) Angstrom and Os-N-N angle of 172.5(4) degrees in trans-[Os-VI(4 ' -O(CH2)(4)Ntpy)(CI)(2)(NN(CH2)(4)O)](2+) are consistent with sp-hybridization of the alpha -nitrogen of the hydrazido ligand and an Os-N triple bond. The extensive ring substitution chemistry implied for the Os(Vl)-hydrazido complexes is discussed.