Phosphonium ylides are chemical compounds characterized by a phosphorus atom bonded to a carbon atom that carries a negative charge. These compounds typically have the general structure R3P=CR2, where R represents organic groups. Phosphonium ylides are important intermediates in organic synthesis, particularly in the Wittig reaction, which is used to form alkenes from aldehydes or ketones. The unique properties of phosphonium ylides arise from their ability to stabilize the negative charge on the carbon atom due to the electron-withdrawing nature of the phosphorus atom. This stability allows them to react with electrophiles, making them valuable in the synthesis of various organic molecules, including pharmaceuticals and agrochemicals.