superoxide (O2−) and peroxide (O22−). Ligands with extended pi-delocalization such as porphyrins, phthalocyanines, and corroles and ligands with the generalised formulas [D-CR=CR-D]n− (D = O, S, NR’ and R, R' = alkyl or aryl) are often non-innocent. In contrast, [D-CR=CR-CR=D]− such as NacNac or acac are innocent. catecholates and related 1,2-dioxalenes. dithiolenes, such as maleonitriledithiolate (see example of [Ni(S2C2Ph2)2]n− above). 1,2-diimines such as derivatives of 1,2-diamidobenzene, 2,2'-bipyridine, and dimethylglyoxime. The complex Cr(2,2'-bipyridine)3 is a derivative of Cr(III) bound to three bipyridine1− ligands. On the other hand, one-electron oxidation of [Ru(2,2'-bipyridine)3]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 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. [4Fe-4S] clusters. The additional redox equivalents provided by redox non-innocent ligands are also used as controlling factors to steer homogeneous catalysis. Hemes Porphyrin ligands can be innocent (2-) or noninnocent (1-). 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. Galactose oxidase The catalytic cycle of galactose oxidase (GOase) 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(II/I) 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. See also Electromerism Isomerism Chiral molecules Redox References Further reading Ligands Organometallic chemistry Chemical bonding