Pyridine complexes of mercury(ii)halides: Implications of a soft metal center for crystal engineering.

The syntheses and crystal structures of eleven new Hg(II) complexes with halogen (chloro, bromo, iodo) and 3,5-disubstituted pyridine ligands are reported. The eight chain polymers among these compounds comprise examples of the hitherto rarely studied class of 1-D polymers based on divalent mercury bridged by the heavier halides Br and I. In the coordination polymers, examples of both pseudooctahedral Hg(II) coordination and of five-coordinated mercury have been found; ligand-size criteria alone cannot account for the structure type adopted. The study reveals that the Hg⋯halogen distances cover the whole range between coordinative bonds and van-der-Waals interactions, ligand geometries around the Hg(II) cations being more asymmetric than in related cadmium complexes. In addition to 1-D polymers, two mononuclear tetrahedral complexes and an example of an extended structure involving Hg–Cl–Hg bridges in two dimensions are described.

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