Phosphonoacetic Acid (H₃AP) and its Salts KH₂AP.H₂ O, (NH₄)H₂AP, LiH₂AP, NaH₂AP.2H₂ O, K₂HAP, (NH₄)₂HAP, Na₂HAP.2H₂ O, (NH₄)₃AP.2H₂ O and Na₃AP.10H₂O

The crystal structures of phosphonoacetic acid, C₂H₅O₅P [H₃AP, (I)], and its salts potassium phosphonoacetate hydrate, K⁺.C₂H₄O₅P⁻.H₂O [KH₂AP.H₂O, two crystalline forms, α, (II), and β, (III)], ammonium phosphonoacetate, NH₄⁺.C₂H₄O₅P⁻ [(NH₄)H₂AP, (IV)], lithium phosphonoacetate, Li⁺.C₂H₄O₅P⁻ [LiH₂AP, (V)], sodium phosphonoacetate dihydrate, Na⁺.C₂H₄O₅P⁻.2H₂O [NaH₂AP.2H₂O, (VI)], dipotassium phosphonoacetate, 2 K⁺.C₂H₃O₅P²⁻ [K₂HAP, (VII)], diammonium phosphonoacetate, 2NH₄⁺.C₂H₃O₅P²⁻ [(NH₄)₂HAP, (VIII)], disodium phosphono­acetate dihydrate, 2Na⁺.C₂H₃O₅P²⁻.2H₂O [Na₂HAP.2H₂O, (IX)], triammonium phosphonoacetate dihydrate, 3NH₄⁺.C₂H₂O₅P³⁻.2H₂O [(NH₄)₃AP.2H₂O, (X)], and trisodium phosphonoacetate decahydrate, 3Na⁺.C₂H₂O₅P³⁻.10H₂O [Na₃AP.10H₂O, (XI)], are described. In all cases, one of the phosphonate O atoms (denoted O2) is nearly antiperiplanar to the carboxylic C1 atom. There is also an evident tendency for the plane through the carboxyl group to be almost perpendicular to the plane through the methylene C atom C2, P and O2. In the acid anions the dihedral angles between these planes range from 75.6 (1)° in (V) to 89.0 (1)° in (VII) whereas in the free acid, (I), this angle is 50.6 (1)°.

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