Tetramethylphosphonium bromide is an organophosphorus compound with the formula (CH<sub>3</sub>)<sub>4</sub>PBr. It is a white, water-soluble solid, the salt of the cation tetramethylphosphonium and the bromide anion. It is prepared by treating trimethylphosphine with methyl bromide.
Deprotonation gives methylenetrimethylphosphine ylide, which can sustain a second deprotonation:
The latter is a precursor to many coordination complexes, e.g., the dicuprous complex Cu<sub>2</sub>[(Me<sub>2</sub>P(CH<sub>2</sub>)<sub>2</sub>]<sub>2</sub>.