Fischer carbenes - structure - bonding - synthesis - NTA CSIR NET IIT JAM GATE Chemistry

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Fischer carbenes.

The Fischer carbenes are the organometallic complexes containing a carbene ligand coordinated to the metal center.

The bonding between the metal center and the carbene ligand involves the formation of a strong sigma bond, where the carbene donates a lone pair of electrons to the metal.

Simultaneously, a weaker pi bond is formed due to the back donation of an electron pair from the metal center to the carbene.

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The carbenes are the divalent carbon species with two unshared electrons. The carbene carbon exhibits a sextet configuration in its valence shell. Therefore, it is a highly reactive species.
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The unshared electrons may be unpaired or paired up.
The carbene with unpaired electrons is referred to as triplet carbene while that with paired electrons is called singlet carbene.
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It case of fischer's carbenes, it is the singlet carbene.
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The singlet carbene makes a strong sigma bond by donating electrons to the metal center and at the same time a weaker pi bond is formed due to the back donation of electron pair from the metal center to the carbene carbon.
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This is the orbital representation of the sigma and pi bond formation between the metal center and the carbene carbon.
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Typically, a Fischer carbene is a hybrid of the following resonance structures. Hence the M=C bond length is greater than expected value.
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The X in the fischer carbene may be an electronegative heteroatom like O, S, N etc. As a result, the carbene carbon gets a more positive charge. Therefore it is electrophilic in nature. It is easily attacked by a nucleophile.
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The metals that form Fischer carbenes are electron-rich and are in the lower oxidation state. Usually, mid to late transition metals like Fe, Co, Mo tend to form Fischer carbenes.
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This is one example of the preparation of a Fischer carbene.