The N—H…O hydrogen bond is the characteristic interaction in the crystal structures of N-benzyl-P-phenyl-N'-(p-tolyl)phosphonic diamide, C20H21N2OP or (C6H5)P(O)(NHCH2C6H5)(NHC6H4-p-CH3), (I), diphenylphosphinic 1-methylpropylamide, C16H20NOP or (C6H5)2P(O)[NHCH(CH3)(C2H5)], (II), (S)-1-phenylethylammonium N-[(S)-1-phenylethyl]phenylphosphonamidate, C8H12N+C14H15NO2P– or [S-(C6H5)CH(CH3)NH3][(C6H5)P(O){S-NHCH(CH3)(C6H5)}(O)], (III), and (4-methylbenzyl)ammonium diphenylphosphinate, C8H12N+C12H10O2P– or [4-CH3-C6H4CH2NH3][(C6H5)2P(O)(O)], (IV). This article focuses on the N—H…O hydrogen bonds by considering the structures of (I), (II), (III) and (IV), and reviewing their analogous compounds, including 43 (C)P(O)(N)2, 102 (C)2P(O)(N), 31 (C)P(O)(N)(O) and 96 (C)2P(O)(O) structures, deposited in the Cambridge Structural Database (CSD). For the structures with a (C)P(O)(N)2 segment, only neutral hydrogen bonds were found in the CSD. The other three classes of compounds included both neutral and ‘charge-assisted’ hydrogen bonds, and the (C)2P(O)(O) structures were particularly noticeable for a high number of cation–anion compounds. The overall tendencies of N…O distances in neutral and cation–anion compounds were compared. The N—H…O hydrogen-bond angles were also analyzed for the four classes of phosphorus compounds.

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