Redox Reactions of Complexes Containing Innocent Vs. Non-Innocent Ligands
Conventionally, redox reactions of coordination complexes are assumed to be metal-centered. The reduction of MnO4- to MnO42- is described by the change in oxidation state of manganese from 7+ to 6+. The oxide ligands do not change in oxidation state, remaining 2- (a more careful examination of the electronic structure of the redox partners reveals however that the oxide ligands are affected by the redox change). Oxide is an innocent ligand. Another example of conventional metal-centered redox couple is 3+/2+. Ammonia is innocent in this transformation.
A clear example of redox non-innocent behavior of ligands is observed for z, which exists in three oxidation states: z = 2-, 1-, and 0. If the ligands are always considered to be dianionic (as is done in formal oxidation state counting), then z = 0 requires that that nickel has a formal oxidation state of +IV. The formal oxidation state of the central nickel atom therefore ranges from +II to +IV in the above transformations (see Figure). However, the formal oxidation state is different from the real (spectroscopic) oxidation state based on the (spectroscopic) metal d-electron configuration. The stilbene-1,2-dithiolate behaves as a redox non-innocent ligand, and the oxidation processes actually take place at the ligands rather than the metal. This leads to the formation of ligand radical complexes. The charge-neutral complex (z =0) is therefore best described as a Ni2+ derivative of S2C2Ph2-. The diamagnetism of this complex arises from anti-ferromagnetic coupling between the unpaired electrons of the two ligand radicals.
The complex Cr(2,2'-bipyridine)3 is a derivative of Cr(III) bound to three radical anions of 2,2'bipyridine, which is in this case also behaving as a redox non-innocent ligand. On the other hand, one-electron oxidation of 2+ is localized on Ru and the bipyridine is behaving as a normal, innocent ligand in this case.
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