Cyclopentaphosphine is the inorganic compound with the formula (PH)5.

Although only of theoretical interest, (PH)5 is parent of many related cyclic polyphosphines that are the subject of research.

They are white, air-sensitive solids which have good solubility in organic solvents.

The three-membered rings feature bulky substituents, e.g., [tert-BuP]3.

The cyclophosphines can be prepared by several methods, one involves reductive coupling of dichlorophosphines:[2] The structures are complicated by the slow pyramidal inversion at phosphorus(III).