Non-innocent ligand
In chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....

, a (redox) non-innocent ligand is a ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

 in a metal complex where the oxidation state is unclear. Typically, complexes containing non-innocent ligands are redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 active at mild potentials
Electric potential
In classical electromagnetism, the electric potential at a point within a defined space is equal to the electric potential energy at that location divided by the charge there...

. The concept assumes that redox reactions in metal complexes are either metal or ligand localized, which is a simplification, albeit a useful one.

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-
A permanganate is the general name for a chemical compound containing the manganate ion, . Because manganese is in the +7 oxidation state, the permanganate ion is a strong oxidizing agent. The ion has tetrahedral geometry...

 to MnO42-
Potassium manganate
Potassium manganate is the inorganic compound with the formula K2MnO4. This green-colored salt is an intermediate in the industrial synthesis of potassium permanganate , a common chemical...

 is described by the change in oxidation state of manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...

 from 7+ to 6+. The oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....

 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
Cobalt(III) hexammine chloride
Hexamminecobalt chloride is the chemical compound with the formula [Co6]Cl3. This coordination compound is considered an archetypal "Werner complex", named after the pioneer of coordination chemistry, Alfred Werner. This salt consists of [Co6]3+ trications with three Cl− anions...

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

A clear example of redox non-innocent behavior of ligands is observed for [Ni(S2C2Ph2)2]z, which exists in three oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...

s: 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
Diamagnetism is the property of an object which causes it to create a magnetic field in opposition to an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the...

 of this complex arises from anti-ferromagnetic coupling between the unpaired electrons of the two ligand radicals.

The complex Cr(2,2'-bipyridine
2,2'-Bipyridine is a organic compound with the formula . This colorless solid, commonly abbreviated bipy or bpy , is an important isomer of the bipyridine family. It is a bidentate chelating ligand, forming complexes with many transition metals...

)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 [Ru(2,2'-bipyridine)3]2+ is localized on Ru and the bipyridine is behaving as a normal, innocent ligand in this case.


C.K. Jørgenson (Cologny-Geneva) 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 non-innocent ligands have been intensively investigated spectroscopically by the groups of K. Wieghardt (MPI Mülheim a/d Ruhr) and W. Kaim (Stuttgart) over the past years. Quite recently it became obvious that redox non-innocent ligands are not just a spectroscopic curiosity, as the radical reactivity of redox non-innocent ligands was demonstrated to play a crucial role in the mechanism of bio-catalytic processes mediated by several metallo-enzymes (e.g. Gallactose Oxidase, Cytochrome P450, methane mono-oxygenase). More recently, some synthetic research groups have started to systematically investigate the (catalytic) reactivity of transition metal complexes with redox non-innocent ligands in organometallic chemistry.

Typical Ligands that often behave as Redox Non-Innocent Ligands

  • O2 and NO.

Ligands with extended pi-delocalization such as porphyrin
Porphyrins are a group of organic compounds, many naturally occurring. One of the best-known porphyrins is heme, the pigment in red blood cells; heme is a cofactor of the protein hemoglobin. Porphyrins are heterocyclic macrocycles composed of four modified pyrrole subunits interconnected at...

s and phthalocyanine
Phthalocyanine is an intensely blue-green coloured macrocyclic compound that is widely used in dyeing. Phthalocyanines form coordination complexes with most elements of the periodic table...

s, ligands with the generalised formulas [D-CR=CR-D]2- or D=CR-CR=D (D = O, S, NR’ and R, R' = alkyl or aryl
In the context of organic molecules, aryl refers to any functional group or substituent derived from an aromatic ring, be it phenyl, naphthyl, thienyl, indolyl, etc....

), and similar related systems are often non-innocent. For example:
  • dioxalenes, such as catecholates.
  • dithiolene
    Metal dithiolene complexes are complexes containing dithiolene ligands. Dithiolene ligands are unsaturated bidentate ligand wherein the two donor atoms are sulfur. Dithiolenes are often referred to as "metallodithiolenes" or "dithiolene complexes"...

    s, such as 1,2-maleonitriledithiolate
  • diimines such as derivatives of 1,2-diaminobenzene, α-diimines, and dimethylglyoxime
    Dimethylglyoxime is a chemical compound described by the formula CH3CCCH3. This colourless solid is the dioxime derivative of the diketone diacetyl . DmgH2 is used in the analysis of palladium or nickel. Its coordination complexes are of theoretical interest as models for enzymes and as catalysts...

  • pyridine-2,6-diimine ligands (relevant in polymerisation and hydrogenation catalysis).

The pyridine-2,6-diimine ligand can be easily reduced by one or two electrons.

Redox Non-Innocent Ligands in Organometallic Chemistry and Catalysis

In paramagnetic organometallic complexes of Rh and Ir (metallo-radicals), ethene ligands, amido ligands, and (reactive) carbene ligands are sometimes also behaving as 'redox non-innocent' ligands:
  • Solvent coordination to some metallo-radical IrII(ethene) species transfers the spin-density from the metal to the redox non-innocent ethene ligand, after which direct radical coupling reactions with the olefinic ligand radical become possible.

  • Oxidation of certain RhI-amido and IrI-amido complexes does not lead to the expected MII-amido species. Instead the unexpected MI-aminyl radical complexes are formed.

  • Carbene formation from diazo compounds at metallo-radical IrII species unexpectedly leads to formation of 'carbene radicals'. This is a result of the redox non-innocent character of Fischer-type carbenes, where one-electron reduction of the carbene ligand by IrII leads to formation of carbon centered 'carbene radicals' coordinated to IrIII. These 'carbene radicals' reveal interesting radical-type reactivities.

Redox 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
A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...

 proteins, such as myoglobin
Myoglobin is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. It is related to hemoglobin, which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells. The only time myoglobin is found in the...

, ligand-centered redox does not occur and the porphyrin is innocent.

Galactose Oxidase (GOase) provides a seminal example for the involvement of reactive non-innocent ligands in bio-catalytic turnover. GOase converts chemo-selectively primary alcohols with O2 into aldehydes and H2O2, with impressive turnover frequencies. The active site of the enzyme GOase contains a tyrosinyl radical which is 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 alcoholate substrate. Thus, the tyrosinyl radical is a reactive fragment in the catalytic cycle which cooperates with the Cu site. This is essential for the function of the enzyme, because the Cu-ion is only capable of one-electron transformations. It is the interplay of the 1e reactivity of the ligand radical and the 1e reactivity of the metal which makes the overall process possible. The radical abstraction nature of the process makes the process extremely fast.
Anti-ferromagnetic coupling between the unpaired spins of the tyrosine radical ligand and the d9 CuII ion (open-shell singlet ground state) explains the observed diamagnetic nature of the resting state of the enzyme, as was confirmed by synthetic model studies.
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