Influence of Ligand Flexibility on the Electronic Structure of Oxidized NiIII-Phenoxide Complexes

One-electron-oxidized Ni-III-phenoxide complexes with salen-type ligands, [Ni(salen)py(2)](2+) ([1(en)-py](2+)) and [Ni(1,2-salcn)py(2)](2+) ([1(cn)-py](2+)), with a five-membered chelate dinitrogen backbone and [Ni(salpn)py(2)](2+) ([2(pn)-py](2+)) with a six-membered chelate backbone, have been characterized with a combination of experimental and theoretical methods. The five-membered chelate complexes [1(en)-py](2+) and [1(cn)-py](2+) were assigned as Ni-III-phenoxyl radical species, while the six-membered chelate complex [2(pn)-py](2+) was concluded to be a Ni-II-bis(phenoxyl radical) species with metal-centered reduction in the course of the one-electron oxidation of the Ni-III-phenoxide complex [2(pn)-py](+). Thus, the oxidation state of the one-electron-oxidized Ni-III salen-type complexes depends on the chelate ring size of the dinitrogen backbone.