Fosfoniylidyn

Fosfoniylidyn refers to a chemical species characterized by a phosphorus atom doubly bonded to a carbon atom, with a formal charge of +1 on the phosphorus and -1 on the carbon, or a neutral species with a double bond. This can be represented by the resonance structures [R3P=CR2] and [R3P−CR2]. The term is derived from "phosphorus," "oxo" (referring to oxygen, though not present in the ylide itself), and "ylidene" (indicating a divalent carbon atom). However, the "oxo" part is a misnomer as oxygen is not typically a component. These compounds are a class of phosphonium ylides where a carbanionic center is directly attached to a positively charged phosphorus atom through a double bond. The P=C double bond has significant covalent character, but also exhibits polar characteristics due to the electronegativity difference between phosphorus and carbon. Fosfoniylidyns are important intermediates in organic synthesis, notably in the Wittig reaction and related transformations, where they react with aldehydes and ketones to form alkenes. Their reactivity stems from the nucleophilic nature of the carbanionic carbon. The stability of fosfoniylidyns can vary widely depending on the substituents attached to both the phosphorus and carbon atoms. Electron-withdrawing groups on the carbon can stabilize the negative charge, while electron-donating groups on the phosphorus can enhance its positive character.