chemistry

Non-innocent ligand

Non-innocent ligand is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Non-innocent ligand rather than just read about it. In short: In chemistry, a (redox) 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.

Key takeaways

Non-innocent ligand belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Non-innocent ligand to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Non-innocent ligand from memory before moving on to harder problems.

Reference excerpt

In chemistry, a (redox) 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 [Co(NH3)6]3+/[Co(NH3)6]2+. Ammonia is innocent in this transformation.

Redox non-innocent behavior of ligands is illustrated by nickel bis(stilbenedithiolate) ([Ni(S2C2Ph2)2]z). As all bis(1,2-dithiolene) 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 (as is done in formal oxidation state counting), 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 (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), 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 (NO) binds to metals in one of two extreme geometries - bent where NO is treated as a pseudohalide (NO−), and linear, where NO is treated as NO+. Dioxygen can be non-innocent, since it exists in two oxidation states, 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-dioxolenes. 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

Galactose oxidase

See also Electromerism Isomerism Chiral molecules Redox

References

Further reading Dzik, W. I..; Zhang, X. P.; de Bruin, B. (2011). "Redox Noninnocence of Carbene Ligands: Carbene Radicals in (Catalytic) C-C Bond Formation". Inorganic Chemistry. 50 (20): 9896–9903. doi:10.1021/ic200043a. PMID 21520926. Büttner, T.; Geier, J.; Frison, G.; Harmer, J.; Calle, C.; Schweiger, A.; Schönberg, H.; Grützmacher, H. (2005). "A Stable Aminyl Radical Metal Complex". Science. 307. 307 (5707): 235–238. Bibcode:2005Sci...307..235B. doi:10.1126/science.1106070. PMID 15653498. S2CID 6625217. Hetterscheid, D.G.H.; Kaiser, J.; Reijerse, E.; Peters, T.P.J.; Thewissen, S.; Blok, A.N.J.; Smits, J.M.M.; de Gelder, R.; de Bruin, B. (2005). "IrII(ethene): Metal or Carbon Radical?". Journal of the American Chemical Society. 127 (6): 1895–1905. doi:10.1021/ja0439470. hdl:2066/32655. PMID 15701024. Blanchard, S.; Derat, E.; Desage-El Murr, M.; Fensterbank, L.; Malacria, M; Mouriès-Mansuy, V. (2012). "Non-Innocent Ligands: New Opportunities in Iron Catalysis". European Journal of Inorganic Chemistry. 2012 (3): 376–389. doi:10.1002/ejic.201100985. Kaim, W. (2012). "The Shrinking World of Innocent Ligands: Conventional and Non-Conventional Redox-Active Ligands". European Journal of Inorganic Chemistry. 2012 (3): 343–348. doi:10.1002/ejic.201101359.

Illustrations

Worked examples

Example 1 — a first encounter with Non-innocent ligand

Start with the simplest possible case. Write down what Non-innocent ligand claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Non-innocent ligand before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Non-innocent ligand ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Non-innocent ligand

In research: Non-innocent ligand appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Non-innocent ligand in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Non-innocent ligand is common in secondary-school and first-year university syllabi. It links to neighbouring topics Chemical bonding, Ligands, Organometallic chemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Non-innocent ligand outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Non-innocent ligand in 20 minutes

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Frequently asked questions

What is Non-innocent ligand in simple terms?

Why does Non-innocent ligand matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Non-innocent ligand?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Non-innocent ligand.