The phosphorothioic triamides (4-Cl-C6H4CH2NH)3P(S) (I) and (4-CH3-C6H4CH2NH)3P(S) (II) were synthesized to study their coordination behaviors towards Hg(II) cation. The resulting complexes, [(4-Cl-C6H4CH2NH)3P(S)]2Hg2Cl4 (III) and [(4-CH3-C6H4CH2NH)3P(S)]2Hg2Cl4 (IV), are the first examples of mercury complexes with SP(NHR)3 ligands characterized by X-ray crystallography. The presence of NH/CH units and some potential competitor acceptors with moderate/weak acceptor capabilities (sulfur, chlorine and π system) provides the opportunity to study the hydrogen bond elements of (I), (III) and (IV), structurally and of all four compounds theoretically. The strengths of inter- and intramolecular hydrogen bonds were assessed by quantum chemical calculations (Atoms In Molecules (AIM), and Natural Bond Orbital (NBO)), and the internal interactions were further examined by reduced density gradient (RDG) analysis. The differences/similarities of structures were addressed in the view point of geometry, conformations related to flexible substituted benzyl moieties, crowding in the structures and non-bonded contacts, stabilization of four-membered Hg2Cl2 rings and some topics related to solution nuclear magnetic resonance studies (chemical shifts and coupling constants). The similarity of structures investigated allowed for a precise assignment of vibrational frequencies. A survey of the Hg—S=P segment in the Cambridge Structural Database shows that the Hg—S bond lengths in (III) and (IV) are among the smallest Hg—S bonds in the structures with the thiophosphoryl-holding ligands.

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