¹⁹⁹ Hg–³¹P Coupling constants in the diethyl phosphonato-complexes [HgX{(EtO)₂PO}][X = Cl, Br, I, MeCO₂, or (EtO)₂PO] and the crystal structure of the chloro-complex have been determined. Chloro(diethyl phosphonato)-mercury(II) crystallises in the monoclinic system, space group Cc with Z= 16 in a cell of dimensions a= 17·43, b= 17·47, c= 12·59 Å, β= 90·15°. The mercury atoms in two of the four molecules comprising the asymmetric unit have a distorted trigonal-bipyramidal environment, while the co-ordination around the mercury atoms of the other two molecules is distorted octahedral. The mercury atoms forms strong bonds only with one chlorine atom and one phosphorus atom in an approximately digonal arrangement, so that molecules of [HgCl{(EtO)₂PO}] are clearly distinguishable; the average Cl–Hg–P angle is 173·3° and the mean Hg–P and Hg–Cl bond lengths are 2·40 and 2·36 Å, respectively. Hg ⋯ O intermolecular distances involved in the mercury co-ordination range from 2·53–2·91 Å. The Hg–Cl bond is longer than when trans to a chlorine atom. The ¹⁹⁹Hg–³¹P coupling constants also reveal the high trans-influence of the phosphorus ligand. These results are compared with parameters for platinum(II) complexes and it is suggested that the trans-influence of phosphorus ligands does not depend significantly on the availability of d_π-electrons from the metal. The preparation of the complex [Hg(NCO)-{(EtO)₂PO}] is described.