Mechanism and molecular-electronic structure correlations in a novel series of Osmium(V) hydrazido complexes

Reaction between the Os(VI) nitrido (Os-VI=N+) complexes [Os-VI(L-3)(Cl)(2) (N)](+) (L-3 is 2,2':6',2 "-terpyridine (tpy) or tris(1-pyrazolyl)methane ( tpm)) and secondary amines (HN(CH2)(4)O = morpholine, HN(CH2)(4)CH2 = piper idine, and HN(C2H5)(2) = diethylamine) gives Os(V)-hydrazido complexes, [Os -V(L-3)(Cl)(2)(NNR2)](+) (NR2 = morpholide, piperidide, or diethylamide). T hey can be chemically or electrochemically oxidized to Os(VI) or reduced to Os(IV) and Os(III). The Os-N bond lengths and Os-N-N angles in the structu res of these complexes are used to rationalize the bonding between the dian ionic hydrazido Ligand and Os. The rate law for formation of the Os(V) hydr azido complexes with morpholine as the base is first order in [Os-VI(L-3)(C l)(2)(N)](+) and second order in HN(CH2)(4)O with k(tpy)(25 degrees C, CH3C N) = (58.1 +/- 1.2) M-2 S-1 and k(tpm)(25 degrees C, CH3CN) = 268.3 +/- 4.0 M-2 s(-1). The proposed mechanism involves initial nucleophilic attack of the secondary amine on the Os(VI) nitrido group to give a protonated Os(IV) -hydrazido intermediate. It is subsequently deprotonated and then oxidized by Os-VI=N+ to Os(V). The extensive redox chemistry for these complexes can be explained by invoking a generalized bonding model. It can also be used to assign absorption bands that appear in the electronic from the visible-n ear-infrared spectra including a series of d pi --> d pi interconfiguration al bands at low energy.