In the hydrogen-bond patterns of phenyl bis(2-chlorobenzylamido)phosphinate, C20H19Cl2N2O2P, (I), and N,N\'-bis(2-chlorobenzyl)-N\"-(2,2,2-trifluoroacetyl)phosphoric triamide, C16H15Cl2F3N3O2P, (II), the O atoms of the related phosphoryl groups act as double H-atom acceptors, so that the P═O…(H—N)2 hydrogen bond in (I) and the P═O…(H—Namide)2 and C═O…H—NC(O)NHP(O) hydrogen bonds in (II) are responsible for the aggregation of the molecules in the crystal packing. The presence of a double H-atom acceptor centre is a result of the involvement of a greater number of H-atom donor sites with a smaller number of H-atom acceptor sites in the hydrogen-bonding interactions. This article also reviews structures having a P(O)NH group, with the aim of finding similar three-centre hydrogen bonds in the packing of phosphoramidate compounds. This analysis shows that the factors affecting the preference of the above-mentioned O atom to act as a double H-atom acceptor are: (i) a higher number of H-atom donor sites relative to H-atom acceptor centres in molecules with P(═O)(NH)3, (N)P(═O)(NH)2, C(═O)NHP(═O)(NH)2 and (NH)2P(═O)OP(═O)(NH)2 groups, and (ii) the remarkable H-atom acceptability of this atom relative to the other acceptor centre(s) in molecules containing an OP(═O)(NH)2 group, with the explanation that the N atom bound to the P atom in almost all of the structures found does not take part in hydrogen bonding as an acceptor. Moreover, the differences in the H-atom acceptability of the phosphoryl O atom relative to the O atom of the alkoxy or phenoxy groups in amidophosphoric acid esters may be illustrated by considering the molecular packing of compounds having (O)2P(═O)(NH) and (O)P(═O)(NH)(N) groups, in which the unique N—H unit in the abovementioned molecules almost always selects the phosphoryl O atom as a partner in forming hydrogen-bond interactions. The P atoms in (I) and (II) are in tetrahedral coordination environments, and the phosphoryl and carbonyl groups in (II) are anti with respect to each other (the P and C groups are separated by one N atom). In the crystal structures of (I) and (II), adjacent molecules are linked via the above-mentioned hydrogen bonds into a linear arrangement parallel to [100] in both cases, in (I) by forming R22(8) rings and in (II) through a combination of R22(10) and R21(6) rings.

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