Organometallic compounds have a bond between a metal atom and a carbon atom from an organic group. Organometallic compounds mainly act as catalysts, speeding up reactions without being consumed themselves. Fischer carbenes and Schrock carbenes are two such organometallic compounds.
What is the difference between Fischer and Schrock carbene? Fischer carbene involves a weak back bonding from the metal to the carbene carbon, while Schrock carbene involves a strong back bonding from the metal to the carbene carbon.
Key Areas Covered
1. What is Fischer Carbene
– Definition, Features
2. What is Schrock Carbene
– Definition, Features
3. Similarities Between Fischer and Schrock Carbene
– Outline of Common Features
4. Difference Between Fischer and Schrock Carbene
– Comparison of Key Differences
5. FAQ: Fischer and Schrock Carbene
– Answers to Frequently Asked Questions
Key Terms
Fischer Carbene, Schrock Carbene
What is Fischer Carbene
Fischer carbene is a type of organometallic compound with the metal center bonding to a special ligand-carbene. This carbene has a unique double bond between carbon and a single other element, typically oxygen or nitrogen. Weak back bonding between the metal and the carbene carbon is the prominent feature of Fischer carbene. The carbene carbon behaves similarly to the carbon in a carbonyl group (C=O) found in many organic molecules. This allows Fischer carbenes to participate in reactions such as transesterification, Michael addition, and aldol-type condensations.
There are many uses of Fischer carbenes. Cycloaddition reactions involve forming cyclic structures by adding a carbene to an unsaturated molecule. The electrophilic nature of Fischer carbenes is mainly useful in this situation. The ability to insert into C-H bonds and participate in various other reactions allows Fischer carbenes to be involved in the synthesis of organic molecules. Understanding the behavior of this molecule helps in understanding the behavior of similar molecules in organic chemistry.
What is Schrock Carbene
Schrock carbenes are a type of organometallic compound. Unlike Fischer carbenes, Schrock carbenes involve a strong back bonding from the metal to the carbene carbon. In Schrock carbenes, the metal center forms a double bond with a carbon atom that also holds a single bond with a hydrogen atom (M=C-H). This arrangement gives the carbon a nucleophilic character (nucleophilic means the ability to readily donate electrons).
In olefin metathesis, Schrock carbene acts as the catalyst, breaking and reforming double bonds between carbon-carbon atoms in olefins. This reshuffling creates new molecules; hence it is involved in organic synthesis. Schrock carbenes are water and air sensitive.
Similarities Between Fischer and Schrock Carbene
- Both types of carbenes feature a double bond between a transition metal center and a carbon atom.
- The metal and carbon atom participate in π-bonding, sharing electrons in orbitals above and below the plane formed by the metal, carbon, and the substituent on the carbon.
- Moreover, Fischer and Schrock carbenes are classified as organometallic complexes, where organic fragments are bound to a central metal atom.
- Both have been also explored for their catalytic potential in various organic transformations.
Difference Between Fischer and Schrock Carbene
Definition
- Fischer carbene is an organometallic compound with a transition metal center bonded to a carbene ligand, whereas Schrock carbene is an organometallic compound where a metal center forms a double bond with a carbene carbon.
Structure
- Fischer carbene involves a weak back bonding from the metal to the carbene carbon, while Schrock carbene involves a strong back bonding from the metal to the carbene carbon.
HOMO and LUMO
- The highest occupied molecular orbital (HOMO) is primarily metal-based, while the lowest unoccupied molecular orbital (LUMO) is ligand-based (often the carbene carbon) in Fischer carbene. The HOMO is mainly ligand-based (the carbene carbon), and the LUMO is metal-based in Schrock carbene.
Reactions
- Fischer carbenes readily react with nucleophiles (electron-rich species) at the carbene carbon, while Schrock carbenes are attacked by electrophiles.
Conclusion
Fischer and Schrock carbenes are both organometallic compounds with a metal-carbene double bond. Fischer carbenes have weak back bonding while Schrock carbenes have a strong back bonding. Thus, this is the main difference between Fischer and Schrock carbene.
FAQ: Fischer and Schrock Carbene
1. What is Fischer type carbene?
Fischer carbene is an organometallic compound having a transition metal centre bonded to a carbene ligand. The metal in a Fischer carbene complex is usually in a low oxidation state. Common metals include iron (Fe), chromium (Cr), molybdenum (Mo), and tungsten (W).
2. What is the reactivity of Fischer and Schrock carbenes?
As electrophiles, Fischer carbenes are susceptible to attack by nucleophiles. Due to their nucleophilic nature, Schrock carbenes react with electrophiles.
3. Which carbene is more stable?
Generally, Fischer carbenes with lower metal oxidation states tend to be more stable compared to Schrock carbenes with higher oxidation states.
4. What does Fischer carbene contain?
A Fischer carbene contains two main parts. They are the metal center having empty d orbitals that can participate in bonding and the carbene ligand which contains a divalent carbon atom with two substituents and a lone pair of electrons.
5. Is carbene a Lewis acid or base?
Carbenes can act as both Lewis acids and bases, depending on their structure. Generally, singlet carbenes are Lewis acids, but there is an exception where singlet carbenes with a filled p orbital can act as Lewis bases.
Reference:
1. “Schrock Carbene.” Encyclopedia Britannica.
2. “Fischer Carbene.” Encyclopedia Britannica.
Image Courtesy:
1. “Fisher Carbene 2” By NatMath13 – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Schrock Orbitals” By NatMath13 – Own work (CC BY-SA 4.0) via Commons Wikimedia
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