For structures with a C2PO2 skeleton, a comparison of “single-and-half” phosphorus–oxygen bonds with single and double phosphorus–oxygen bonds is presented. The study is performed by considering the title structure, 63 analogous salt/zwitterionic structures with the [O–P=O]– moiety and 445 structures with a neutral O–P=O moiety retrieved from the Cambridge Structural Database. The average lengths of the single, “single-and-half” and double phosphorus-oxygen bonds were obtained as 1.58, 1.50, and 1.48 Å, which indicates the proximity of “single-and-half” bond lengths to double phosphorus–oxygen bond lengths. In the crystal structure of the title compound, the [NH2C5H4NH]+ cation, [(C6H5)2P(O)(O)]– anion and two independent water molecules are interconnected through relatively strong O–H⋅⋅⋅O and N–H⋅⋅⋅O hydrogen bonds, building a two-dimensional assembly along the ab plane. In this hydrogen-bond pattern, the oxygen atoms of the [O–P=O]– moiety act as double hydrogen-bond acceptors.

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