. Stabilization of trivalent nickel through 1:2 coordination by cyclic terdentate ligands CH2CH2NH(CH2)2NH(CH2)2X (X = NH, O, or S)

The octahedral 6-coordinated complexes [Ni(L)2]2+ (L = 1-oxa-4,7-diazacyclononane; L1 = 1-thia-4,7-diazacyclononane) (donor set: N4O2 and N4S2, resp.) undergo a reversible 1-electron oxidn. in MeCN and DMSO solns. to give authentic Ni(III) species. These Ni(III) complexes present a distorted stereochem. arrangement, with the O and S heteroatoms occupying the axial sites of an elongated octahedron, and they persist in soln. for hours. Comparison with the corresponding complexes of 1,4,7-triazacyclonanane (L2) (donor set: N6) indicates that the relative stability of the trivalent species, expressed by the value of E1/2(Ni(III)-Ni(II)), decreases along the series: L2 > L1 > L, the trend reflecting the decrease in the strength of the Ni-X interactions (X = N, S, or O). The corresponding Ni(III) complexes of the open-chain analogs of the macrocycles L3 or L1 form at comparable potentials, but persist in soln. only in the time-scale of the voltammetry expt., thus demonstrating the existence of a purely kinetic macrocyclic effect in the stabilization of high oxidn. states of metals.