Ylidene

Ylidene refers to a divalent organic group in which two substituents are attached to the same carbon atom, and this carbon atom is doubly bonded to another atom. The term is often used in the context of carbenes and related species, where the carbon atom forms a double bond with another element.

A common example of an ylidene is in a Fischer carbene complex. In these complexes, a metal atom is bonded to a carbon atom, and this carbon atom is also bonded to two organic substituents and is doubly bonded to another atom, typically oxygen or sulfur. The general structure can be represented as M=C(R1)(R2)X, where M is a metal, R1 and R2 are organic groups, and X is usually an oxygen or sulfur atom.

Another context where the term ylidene appears is in ylide chemistry. While an ylide is formally a neutral molecule with adjacent positive and negative charges, the carbon atom bearing the negative charge is often referred to as a carbanionic center. If this carbanionic center is involved in a double bond to another atom, the resulting species could be considered an ylidene. However, the term ylidene is more specifically associated with the divalent carbon in a double bond context. The electronic structure of ylidenes can vary, and they are often reactive intermediates or components of more complex molecules.