Non-innocent ligand
In chemistry, a non-innocent ligand is a ligand in a metal complex where the oxidation state is not
clear. 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.
C.K. Jørgensen first 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..."
Redox reactions of complexes of 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. Oxide is an innocent ligand. Another example of conventional metal-centered redox couple isCobalt hexammine chloride|
Redox non-innocent behavior of ligands is illustrated by nickel bis. As all bis complexes of nd8 metal ions, three oxidation states can be identified: z = −2, −1, and 0. If the ligands are always considered to be dianionic, then z = 0 requires that that nickel has a formal oxidation state of +4. The formal oxidation state of the central nickel atom therefore ranges from +2 to +4 in the above transformations. However, the formal oxidation state is different from the real oxidation state based on the 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, showing a partial singlet diradical character, is therefore better described as a Ni2+ derivative of the radical anion S2C2Ph2•−. The diamagnetism of this complex arises from anti-ferromagnetic coupling between the unpaired electrons of the two ligand radicals.
Another example is higher oxidation states of copper complexes of diamido phenyl ligands that are stabilized by intramolecular multi center hydrogen bonding
Typical non-innocent ligands
- Nitrosyl binds to metals in one of two extreme geometries - bent where NO is treated as a pseudohalide, and linear, where NO is treated as NO+.
- Dioxygen can be non-innocent, since it exists in two oxidation states, superoxide and peroxide.
- catecholates and related 1,2-dioxolenes.
- dithiolenes, such as maleonitriledithiolate.
- 1,2-diimines such as derivatives of 1,2-diamidobenzene, 2,2'-bipyridine, and dimethylglyoxime. The complex Cr3 is a derivative of Cr bound to three bipyridine1− ligands. On the other hand, one-electron oxidation of Ruthenium tris chloride|
2+ is localized on Ru and the bipyridine is behaving as a normal, innocent ligand in this case. - ligands containing ferrocene can have oxidation events centered on the ferrocene iron center rather than the catalytically active metal center.
- pyridine-2,6-diimine ligands can be reduced by one and two electrons.
Redox non-innocent ligands in biology and homogeneous catalysis
Hemes
:
Porphyrin ligands can be innocent or noninnocent. In the enzymes chloroperoxidase and cytochrome P450, the porphyrin ligand sustains oxidation during the catalytic cycle, notably in the formation of Compound I. In other heme proteins, such as myoglobin, ligand-centered redox does not occur and the porphyrin is innocent.