chemistry

Transition metal carbene complex

Transition metal carbene complex 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 Transition metal carbene complex rather than just read about it. In short: A transition metal carbene complex is an organometallic compound featuring a divalent carbon ligand, itself also called a carbene. Carbene complexes have been synthesized from most transition metals and f-block metals, using many different synthetic routes such as nucleophilic addition and alpha-hydrogen abstraction.

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Reference excerpt

A transition metal carbene complex is an organometallic compound featuring a divalent carbon ligand, itself also called a carbene. Carbene complexes have been synthesized from most transition metals and f-block metals, using many different synthetic routes such as nucleophilic addition and alpha-hydrogen abstraction. The term carbene ligand is a formalism since many are not directly derived from carbenes and most are much less reactive than lone carbenes. Described often as =CR2, carbene ligands are intermediate between alkyls (−CR3) and carbynes (≡CR). Many different carbene-based reagents such as Tebbe's reagent are used in synthesis. They also feature in catalytic reactions, especially alkene metathesis, and are of value in both industrial heterogeneous and in homogeneous catalysis for laboratory- and industrial-scale preparation of fine chemicals.

Classification Metal carbene complexes are often classified into two types. The Fischer carbenes, named after Ernst Otto Fischer, feature strong π-acceptors at the metal and are electrophilic at the carbene carbon atom. Schrock carbenes, named after Richard R. Schrock, are characterized by more nucleophilic carbene carbon centers; these species typically feature higher oxidation state (valency) metals. N-Heterocyclic carbenes (NHCs) were popularized following Arduengo's isolation of a stable free carbene in 1991. Reflecting the growth of the area, carbene complexes are now known with a broad range of different reactivities and diverse substituents. Often it is not possible to classify a carbene complex solely with regards to its electrophilicity or nucleophilicity.

Fischer carbenes

The common features of Fischer carbenes are:

low oxidation state metal center middle and late transition metals Fe(0), Mo(0), Cr(0) π-acceptor metal ligands π-donor substituents on the carbene atom such as alkoxy and alkylated amino groups. Examples include (CO)5W=COMePh and (OC)5Cr=C(NR2)Ph.

Fischer carbene complexes are related to the singlet form of carbenes, where both electrons occupy the same sp2 orbital at the carbon. This lone pair donates to a metal-based empty d orbital, forming a σ bond. π-backbonding from a filled metal d orbital to the empty p orbital of the carbon atom is possible. However this interaction is generally weak since the alpha donor atoms also donate to this orbital. As such, Fischer carbenes are characterized as having partial double bond character. The major resonance structures of Fischer carbenes put the negative charge on the metal centre, and the positive on the carbon atom, making it electrophilic.

Fischer carbenes can be likened to ketones, with the carbene carbon atom being electrophilic, like the carbonyl carbon atom of a ketone. This can be seen from the resonance structures, where there is a significant contribution from the structure bearing a positive carbon centre. Like ketones, Fischer carbene species can undergo aldol-like reactions. The hydrogen atoms attached to the carbon atom α to the carbene carbon atom are acidic, and can be deprotonated by a base such as n-butyllithium, to give a nucleophile, which can undergo further reaction.

Schrock carbenes

Schrock carbenes do not have π-accepting ligands on the metal centre. They are often called alkylidene complexes. Typically this subset of carbene complexes are found with:

high oxidation state metal center early transition metals Ti(IV), Ta(V) σ-donor and sometimes π-donor metal ligands hydrogen and alkyl substituents on carbenoid carbon. Examples include ((CH3)3CCH2)Ta=CHC(CH3)3 and Os(PPh3)2(NO)Cl(=CH2).

Bonding in such complexes can be viewed as the coupling of a triplet state metal and triplet carbene, forming a true double bond. Both the metal and carbon atom donate 2 electrons, one to each bond. Since there is no donation to the carbene atom from adjacent groups, the extent of pi backbonding is much greater, giving a strong double bond. These bonds are weakly polarized towards carbon and therefore the carbene atom is a nucleophile. Furthermore, the major resonance structures of Schrock carbene put the negative charge on the carbon atom, making it nucleophilic. Complexes with the methylidene ligand (=CH2) are the simplest Schrock-type carbenes.

N-Heterocyclic carbenes

N-Heterocyclic carbenes (NHCs) are particularly common carbene ligands. They are popular because they are more readily prepared than Schrock and Fischer carbenes. In fact, many NHCs are isolated as the free ligand, since they are persistent carbenes. Being strongly stabilized by π-donating substituents, NHCs are powerful σ-donors but π-bonding with the metal is weak. For this reason, the bond between the carbon and the metal center is often represented by a single dative bond, whereas Fischer and Schrock carbenes are usually depicted with double bonds to metal. Continuing with this analogy, NHCs are often compared with trialkylphosphine ligands. Like phosphines, NHCs serve as spectator ligands that influence catalysis through a combination of electronic and steric effects, but they do not directly bind substrates.

Bimetallic carbene complexes An early example of this bonding mode was provided by [C5Me5Mn(CO)2]2(μ−CO) prepared from diazomethane:

2 C5Me5Mn(CO)2(thf) + CH2N2 → [C5Me5Mn(CO)2]2(μ−CH2] + N2 + 2 thf Another example of this family of compounds is Tebbe's reagent. It features a methylene bridge joining titanium and aluminum.

Application of Metal Carbenes Metal carbene complexes have applications in hetereogeneous and homogeneous catalysis, and as reagents for organic reactions.

Catalysis

The dominant application of metal carbenes involves none of the above classes of compounds, but rather heterogeneous catalysts used for alkene metathesis for the synthesis of higher alkenes. A variety of related reactions are used to interconvert light alkenes, e.g. butenes, propylene, and ethylene. Carbene complexes are invoked as intermediates in the Fischer–Tropsch route to hydrocarbons. A variety of homogeneous carbene catalysts, especially the Grubbs' ruthenium and Schrock molybdenum-imido catalysts have been used for olefin metathesis in laboratory-scale synthesis of natural products and materials science.

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Illustrations

Transition metal carbene complex: Major resonance structures of (CO)5W=COMePh. Structures with a positive charge on carbon are significant and make the carbon electrophilic. — Major resonance structures of (CO)5W=COMePh. Structures with a positive charge on carbon are significant and make the carbon electrophilic.

Transition metal carbene complex: Major resonance structures of a Schrock carbene. The negative charge at the carbon atom renders it nucleophilic. — Major resonance structures of a Schrock carbene. The negative charge at the carbon atom renders it nucleophilic.

Worked examples

Example 1 — a first encounter with Transition metal carbene complex

Start with the simplest possible case. Write down what Transition metal carbene complex 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 Transition metal carbene complex 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 Transition metal carbene complex 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 Transition metal carbene complex

In research: Transition metal carbene complex 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 Transition metal carbene complex 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: Transition metal carbene complex is common in secondary-school and first-year university syllabi. It links to neighbouring topics Carbenes, Coordination complexes, Organometallic chemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Transition metal carbene complex 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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Frequently asked questions

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