Phosphine oxides (designation σλ) have the general structure RP=O with formal oxidation state V. Phosphine oxides form hydrogen bonds and some are therefore soluble in water. The P=O bond is very polar with a dipole moment of 4.51 D for triphenylphosphine oxide.
Compounds related to phosphine oxides include phosphine imides (RPNR') and related chalcogenides (RPE, where E = S, Se, Te).  These compounds are some of the most thermally stable organophosphorus compounds.