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 is Cobalt hexammine chloride|3+/2+. Ammonia is innocent in this transformation. Redox non-innocent behavior of ligands is illustrated by nickel bis, which exists in three oxidation states: 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 +IV. The formal oxidation state of the central nickel atom therefore ranges from +II to +IV 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 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. 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.
Ligands with extended pi-delocalization such as porphyrins, phthalocyanines, and corroles and ligands with the generalised formulas n− are often non-innocent. In contrast, − such as NacNac or acac are innocent.
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.
In certain enzymatic processes, redox non-innocent cofactors provide redox equivalents to complement the redox properties of metalloenzymes. Of course, most redox reactions in nature involve innocent systems, e.g. ferrodoxin| clusters. The additional redox equivalents provided by redox non-innocent ligands are also used as controlling factors to steer homogeneous catalysis.
The catalytic cycle of galactose oxidase illustrates the involvement of non-innocent ligands. GOase oxidizes primary alcohols into aldehydes using O2 and releasing H2O2. The active site of the enzyme GOase features a tyrosyl coordinated to a CuII ion. In the key steps of the catalytic cycle, a cooperative Brønsted-basic ligand-site deprotonates the alcohol, and subsequently the oxygen atom of the tyrosinyl radical abstracts a hydrogen atom from the alpha-CH functionality of the coordinated alkoxide substrate. The tyrosinyl radical participates in the catalytic cycle: 1e-oxidation is effected by the Cu couple and the 1e oxidation is effected by the tyrosyl radical, giving an overall 2e change. The radical abstraction is fast. Anti-ferromagnetic coupling between the unpaired spins of the tyrosine radical ligand and the d9 CuII center gives rise to the diamagnetic ground state, consistent with synthetic models.
