A phosphonium ylide is a type of chemical compound that contains a phosphorus atom bonded to a carbon atom, which carries a negative charge. This structure is characterized by a positively charged phosphonium group and a negatively charged carbon atom, making it a zwitterionic species. Phosphonium ylides are often used in organic synthesis, particularly in the Wittig reaction, where they help form alkenes by reacting with carbonyl compounds. These compounds are typically generated by deprotonating a phosphonium salt, which results in the formation of the ylide. Phosphonium ylides are valuable in the field of organic chemistry due to their ability to facilitate the formation of carbon-carbon double bonds, contributing to the synthesis of various organic molecules.