Non-innocent ligand

In chemistry, a non-innocent ligand refers to a ligand in a metal complex where the oxidation state is unclear. Typically, complexes containing non-innocent ligands are redox active at mild potentials. The concept assumes that redox reactions in metal complexes are either metal or ligand localized, which is a simplification, albeit a useful one.

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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 MnO₄^2- 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 [Co(NH₃)₆]³⁺/[Co(NH₃)₆]²⁺. Ammonia is innocent in this transformation.

[Ni(S₂C₂Ph₂)₂]^z exists in three oxidation states: z = 0, 1-, and 2-. If the ligand are viewed as a dianionic, then z = 0 requires that that the nickel be 4+. The stilbene-1,2-dithiolate is, however, non-innocent, and the charge-neutral complex is described as a Ni²⁺ derivative of S₂C₂Ph₂^-. The diamagnetism of this complex arises from the coupling of the spins on the two ligands.

The complex Cr(2,2'-bipyridine)₃ is a derivative of Cr(III) bound to three radical anions of 2,2'bipyridine, which is non-innocent. On the other hand, one-electron oxidation of [Ru(2,2'-bipyridine)₃]²⁺ is localized on Ru and the bipyridine is innocent.

History

C. K. Jorgenson described ligands as "innocent" and "suspect": "Ligands are innocent when they allow oxidation states of the central atoms to be defined. The simplest case of a suspect ligand is NO..."^[1]

Ligands that are typically non-innocent

Ligands with extended pi-delocalization such as porphyrins and phthalocyanines are often non-innocent. Ligands with the formulas [D-CR=CR-D]^2- or D=CR-CR=D often are non-innocent, where D = O, S, NR’ and R, R' = alkyl or aryl. These ligand classes include:

• dioxalenes, such as catecholates.^[2]
• dithiolenes, such as 1,2-maleonitriledithiolate
• diimines such as derivatives of 1,2-diaminobenzene, α-diimines, and dimethylglyoxime.

Non-innocent ligands in biology

Metalloenzymes often feature non-innocent ligands. A common non-innocent ligand is found in metalloporphyrins. In the enzyme cytochrome P450, the porphyrin ligand sustains oxidation during the catalytic cycle. In other heme proteins, such as myoglobin, ligand-centered redox does not occur and the porphyrin is innocent.

References

1. ^ Jørgensen, Chr. K. (1966). "Differences between the four halide ligands, and discussion remarks on trigonal-bipyramidal complexes, on oxidation states, and on diagonal elements of one-electron energy". Coordination Chemistry Reviews 1: 164-178. doi:10.1016/S0010-8545(00)80170-8.
2. ^ Piero Zanello, P.; Corsini, M. (2006). "Homoleptic, mononuclear transition metal complexes of 1,2-dioxolenes: Updating their electrochemical-to-structural (X-ray) properties". Coordination Chemistry Reviews 250: 2000-2022. doi:10.1016/j.ccr.2005.12.017.