Thermal expansion of mercury(II) cyanide and HgCN(NO3)

Crystals of HgCN(NO3) were prepared by the reaction of Hg(CN)2 in nitric acid. Single crystal X-ray structures of both HgCN(NO3) and Hg(CN)2 were obtained at 100, 205, 295 and 395 K (or 375 K for Hg(CN)2); both structures have been previously reported at room temperature. The unit cells were also determined using powder X-ray diffraction at 15 K intervals between 100 and 395 K. From this data, the coefficients of thermal expansion were obtained. HgCN(NO3) shows positive and negative thermal expansion (PTE and NTE) along the NO3- and CN-based axes respectively, with αa = +26.1(2) × 10−6 K−1 and αc = −23(1) × 10−6 K−1. Hg(CN)2 shows PTE in all directions, with αa = +22.8(2) × 10−6 K−1 and αc = +71.3(5) × 10−6 K−1. This data, which adds to the relatively limited thermal expansion studies on metal cyanide and cyanometallate coordination polymers, should spur further investigation into the thermal behaviour of this class of materials.

Many cyanometallate coordination polymers have yet to be investigated for their thermal expansion behaviour and such fundamental studies on this basic materials property of these complexes are very worthwhile. HgCN(NO3) shows negative and positive thermal expansion (NTE and PTE) along the CN− and NO3− axes. Unusually for a metal-cyanide system, Hg(CN)2 shows PTE in all directions. These phenomena are discussed in terms of the structures of each system.Figure optionsDownload as PowerPoint slide