title: Cyclopentaphosphine
text: # Cyclopentaphosphine
  
  Cyclopentaphosphine is the inorganic compound with the formula $(PH)_5.$ It is prepared by the hydrolysis of $cyclo-[PSiMe_3]_4$ (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:
  
      5 $PhPCl_2$ + 5 Mg → $[PhP]_5$ + 5 $MgCl_2$
  
  ## Isomerism
  
  The structures are complicated by the slow pyramidal inversion at phosphorus(III). In principle, many isomers are possible for $cyclo-P_5R_5,$ but usually only one is observed. All phenyl substituents are equatorial in $cyclo-P_6(C_6H_5)_6.$
  
  