Effect of the size of halide ligands on the crystal structures of halide-bibridged polymers of Hg <i>X</i> ₂ with 4-ethylpyridine

Halide-bridged polymers are a type of coordination polymer whereby halide ligands act as bridging ligands between the metal centres. The crystal structures of three halide-bibridged polymers of the formula [Hg(μ-X)₂(4-Etpy)]_n, namely, catena-poly[[(4-ethylpyridine)mercury(II)]-di-μ-halido], obtained through the combination of the organic ligand 4-ethylpyridine (4-Etpy, C₇H₉N) and HgX₂ (X = Cl, Br or I), were determined. In these structures, abbreviated as 4epHgCl, 4epHgBr and 4epHgI, respectively, the Hg^II ion exhibits a coordination number of five. All three structures were found to display a similar one-dimensional scalloped polymeric chain with halide ligands bridging pairs of Hg^II ions in a bidentate fashion; however, 4epHgI differs from the other two structures in terms of the packing arrangement of the polymer. The change of the halide ligand to the larger iodide ligand disrupts the formation of the regular halide-bibridged polymeric chain observed in the chloride and bromide analogues, with 4epHgI displaying pseudo-bridging in the polymer chain.