Compounds where phosphorus exists in a formal oxidation state of less than III are uncommon, but examples are known for each class.  Organophosphorus(0) species are debatably illustrated by the carbene adducts, [P(NHC)], where NHC is an N-heterocyclic carbene. With the formulae (RP) and (RP), respectively, compounds of phosphorus(I) and (II) are generated by reduction of the related organophosphorus(III) chlorides:
:5 PhPCl  + 5 Mg  →  (PhP)  +  5 MgCl
:2 PhPCl  +  Mg  →  PhP-PPh  +  MgCl
Diphosphenes, with the formula RP, formally contain phosphorus-phosphorus double bonds. These phosphorus(I) species are rare but are stable provided that the organic substituents are large enough to prevent catenation.  Many mixed-valence compounds are known, e.g. the cage P(CH).