From the viewpoint of inorganic crystal engineering (ICE), the coordination sphere of the metal centre can be affected by two main parts of inorganic and organic units in complexes. Database study can play a significant role in the explanation of the relationship between various parameters related to these parts. For the first time, we have investigated this relationship through the concomitant studies of the inorganic crystal structure database (ICSD) and the cambridge structural database (CSD) for mercury(II) halide compounds. The results of CSD analysis are divided into two categories of metal halide complexes (MHC or mercury halide compounds with ligands) and metal halide only (MHO or mercury halide compounds without ligands). MHC (970, 460, and 521 metal centres as HgCl2, HgBr2, and HgI2, respectively) and MHO (419, 141, and 201 metal centres as HgCl2, HgBr2, and HgI2, respectively) were structurally investigated. The coordination number, polymerization mode, coordination geometry of the metal centre, type of donor atom in ligands, and the chelation mode of the ligand for all MHC and MHO compounds were extracted as effective factors in inorganic and organic units. To rationalize the effect of ICE in the design of the coordination sphere, eleven new mercury halide complexes, including the ester ligands of L1, naphthalene-5-yl nicotinate (complexes 1–3), L2, naphthalene-6-yl nicotinate (complexes 4–6), L3, naphthalene-5-yl pyrazine-2-carboxylate (complexes 7–9), and L4, naphthalene-6-yl pyrazine-2-carboxylate (complexes 10–11) were synthesized and fully characterized. The various parameters of substitution, C–H to nitrogen replacement, counteranion, and symmetry effects were investigated for all of the complexes. The results show that there is a meaningful relationship between inorganic and organic units. According to the findings of the CSD and ICSD analyses, most of the complexes obeyed the same relationship. Despite the predominant role of the inorganic unit in determining the coordination geometry, the organic unit can also change the coordination sphere of complexes with one major effect or the cooperativity of minor effects.

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