Metal-Carbon Multiple Bonds

Metal–carbon double and triple bonds in carbene and carbyne complexes range from electrophilic Fischer to nucleophilic Schrock types and provide the active species of olefin metathesis.

Definition

Metal–carbon multiple bonds are double bonds in carbene (alkylidene) complexes and triple bonds in carbyne (alkylidyne) complexes, characterized by metal-to-carbon pi interactions that distinguish them from ordinary metal–alkyl single bonds.

Scope

This topic covers complexes containing metal–carbon multiple bonds: the bonding, reactivity, and contrast between Fischer (low-valent, electrophilic) and Schrock (high-valent, nucleophilic) carbenes (alkylidenes); metal carbyne (alkylidyne) complexes; and the role of metal alkylidenes as catalysts for olefin metathesis. It treats these species as a distinct ligand class within organometallic chemistry, leaving general catalytic cycles to organometallic catalysis.

Core questions

• How does the bonding in carbene complexes differ from a metal–alkyl single bond?
• What distinguishes Fischer from Schrock carbenes in structure and reactivity?
• How do metal alkylidenes catalyse olefin metathesis?
• What are carbyne complexes and how do they bond?

Key concepts

• Metal carbenes (alkylidenes)
• Fischer and Schrock carbenes
• Metal carbynes (alkylidynes)
• Metallacyclobutane intermediates
• Olefin metathesis
• N-heterocyclic carbenes

Key theories

Fischer versus Schrock carbenes

Fischer carbenes are low-valent, heteroatom-stabilized, and electrophilic at carbon, while Schrock carbenes (alkylidenes) are high-valent, lack pi-donor substituents, and are nucleophilic at carbon, reflecting opposite metal–carbon polarities.

Olefin metathesis by metal alkylidenes

Metal alkylidenes interconvert alkenes by reversibly forming and cleaving metallacyclobutane intermediates, the Chauvin mechanism that underlies catalytic olefin metathesis.

Carbyne complexes

Metal carbyne (alkylidyne) complexes contain a formal metal–carbon triple bond and extend the carbene chemistry to higher bond order, with their own Fischer- and Schrock-type variants and metathesis chemistry.

Mechanisms

In olefin metathesis a metal alkylidene reacts with an alkene to form a four-membered metallacyclobutane that fragments in the alternative direction, exchanging the alkylidene partners and thereby scrambling the alkene substituents.

Clinical relevance

Metal–carbon multiple bonds enable olefin metathesis, recognized by the 2005 Nobel Prize, which is used to make pharmaceuticals, polymers, and fine chemicals, and N-heterocyclic carbenes serve as robust supporting ligands across catalysis.

History

Fischer prepared the first stable carbene complex in 1964, and Schrock later characterized the contrasting high-valent alkylidenes. Chauvin's metallacyclobutane mechanism explained olefin metathesis, and the development of well-defined metathesis catalysts by Schrock and Grubbs earned the trio the 2005 Nobel Prize.

Key figures

• Ernst Otto Fischer
• Richard Schrock
• Yves Chauvin
• Robert Grubbs

Seminal works

• fischer1964
• crabtree2014
• hartwig2010

Frequently asked questions

What is the key difference between Fischer and Schrock carbenes?

Fischer carbenes occur with low-valent, electron-rich metals and a heteroatom substituent and are electrophilic at the carbene carbon, whereas Schrock carbenes occur with high-valent, electron-poor metals lacking such stabilization and are nucleophilic at carbon.

How does a metal carbene catalyse olefin metathesis?

The carbene adds across an alkene to give a metallacyclobutane ring; this ring can break apart in the other direction, releasing a new alkene and a new carbene, so repeating the cycle interchanges the substituents on the alkenes.