Redox-induced terpyridyl substitution in the Os(VI)-hydrazido complex, trans-[Os-VI(tpy)(Cl)(2)(NN(CH2)(4)O)](2+)

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- = morpho lide), and a series of N- or O-bases gives as products the substituted Os(V I)-hydrazido complexes, trans-[Os-VI(4 ' -RNtpy)(Cl)(2)(NN(CH2)(4)O)](2+) o r 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-); piperid ide ((CH2)(5)N-); morpholide (O(CH2)(4)N-); ethylamide (EtNH-); diethylamid e (Et2N-); and tert-butylamide (t-BuNH-) and RO- = tert-butoxide (t-BuO-) a nd acetate (MeCO2-). The rate law for the formation of the morpholide-subst ituted 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 substitutio n at the 4 ' -position of the tpy ligand by the added nucleophiles. The key features of the suggested mechanisms are the extraordinary electron withdr awing effect of Os(VI) on tpy and the ability of the metal to undergo intra molecular Os(VI) to Os(IV) electron transfer. These substituted Os(Vl)-hydr azido 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 -nitro gen of the hydrazido ligand and an Os-N triple bond. The extensive ring sub stitution chemistry implied for the Os(Vl)-hydrazido complexes is discussed .