Cyclopentaphosphine
Cyclopentaphosphine is the inorganic compound with the formula (PH)5. It is prepared by the hydrolysis of cyclo-[PSiMe3]4 (Me = methyl). Although only of theoretical interest, (PH)5 is parent of many related cyclic polyphosphines that are the subject of research. Organic cyclophosphines

Organic cyclophosphanes are a family of organophosphorus compounds with the formula (RP)n where R is an organic substituent. Many examples are known. They are white, air-sensitive solids with have good solubility in organic solvents. Well-characterized examples are known for ring sizes 3–6. 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:
5PhPCl2  +  5 Mg  →  [PhP]5  +  5MgCl2

Isomerism
The structures are complicated by the slow pyramidal inversion at phosphorus(III). In principle, many isomers are possible for cyclo-P5R5, but usually only one is observed. All phenyl substituents are equatorial in cyclo-P6(C6H5)6. References

Phosphines
Five-membered rings