Phosphites, sometimes called phosphite esters, have the general structure P(OR) with oxidation state +3. Such species arise from the alcoholysis of phosphorus trichloride:
:PCl  +  3 ROH   →   P(OR)  +  3 HCl
The reaction is general, thus a vast number of such species are known.  Phosphites are employed in the Perkow reaction and the Michaelis–Arbuzov reaction. They also serve as ligands in organometallic chemistry.
Intermediate between phosphites and phosphines are phosphonites (P(OR)R) and phosphinite (P(OR)R). Such species arise via alcoholysis reactions of the corresponding phosphonous and phosphinous chlorides ((PClR) and (PClR) , respectively).  The latter are produced by reaction of a phosphorus trichloride with a poor metal-alkyl complex, e.g. organomercury, organolead, or a mixed lithium-organoaluminum compound.