Question

Which of the following sentences are true?
A.Schrock carbene does nucleophilic reactions
B.Fischer carbene does electrophilic reactions
C.Both A and B
D.None

Answer 1

: To answer this question you must recall the properties of Schrock carbenes and Fischer carbenes. Both Schrock carbenes and Fischer carbenes are examples of transition metal carbene complexes. Schrock carbenes are formed by early transition metals and Fischer carbenes are formed by middle and late transition metals.

Complete step by step answer:
Metal carbene complexes are majorly divided into two types, namely Fischer carbenes and Schrock carbenes.
Schrock carbenes are those which contain a metal center with a high oxidation state. They are formed by early transition metals like titanium and tantalum. They have pi- donor ligands and the carbenoid carbon has only hydrogen or alkyl substituents attached to it. They do not have pi- electrons accepting ligands. The bonding in these carbenes takes place by the coupling of a triplet carbene and a triplet state metal. The bonds are polar and electron density is shifted towards the carbon atom making the complex nucleophilic.
Fischer carbenes are those carbenes which contain a metal center with a lower oxidation state. They are formed by middle and late transition metals like iron, chromium, molybdenum etc. They have pi- acceptor ligands and the carbenoid carbon has pi –donor substituents like alkoxy and alkylate amino groups. The bonding in these carbenes is based on a sigma type electron donation of the lone pair of the carbene atom to the vacant d- orbital of the metal and the back bonding of pi- electron density from metal to the vacant p- orbital of carbon atom. Thus, it is electrophilic.
Both A and B are correct statements.

Thus, the correct option is C.

Note:
Transition metal carbene complexes are organometallic compounds consisting of a divalent organic ligand. This divalent ligand is carbene. Carbene is a neutral molecule having an empty orbital as well as a lone pair of electrons and is bonded to two substituent groups.