Phosphonates feature tetrahedral phosphorus centers. They are structurally closely related to (and often prepared from) phosphorous acid.
Phosphonate salts are the result of deprotonation of phosphonic acids, which are diprotic acids:
:RPO(OH) + NaOH → HO + RPO(OH)(ONa) (monosodium phosphonate)
:RPO(OH)(ONa) + NaOH → HO + RPO(ONa) (disodium phosphonate)
Phosphonate esters are the result of condensation of phosphonic acids with alcohols.