Synthesis, PtS-Type Structure, and Anomalous Mechanics of the Cd(CN)2 Precursor Cd(NH3)2[Cd(CN)4]

We report the nonaqueous synthesis of Cd(CN)<sub>2</sub> by oxidation of cadmium metal with Hg(CN)<sub>2</sub> in liquid ammonia. The reaction proceeds <i>via</i> an intermediate of composition Cd(NH<sub>3</sub>)<sub>2</sub>[Cd(CN)<sub>4</sub>], which converts to Cd(CN)<sub>2</sub> on prolonged heating. Powder X-ray diffraction measurements allow us to determine the crystal structure of the previously-unreported Cd(NH<sub>3</sub>)<sub>2</sub>[Cd(CN)<sub>4</sub>], which we find to adopt a twofold interpenetrating PtS topology. We discuss the effect of partial oxidation on the Cd/Hg composition of this intermediate, as well as its implications for the reconstructive nature of the deammination process. Variable-temperature X-ray diffraction measurements allow us to characterise the anisotropic negative thermal expansion (NTE) behaviour of Cd(NH<sub>3</sub>)<sub>2</sub>[Cd(CN)<sub>4</sub>] together with the effect of Cd/Hg substitution; <i>ab initio</i> density functional theory (DFT) calculations reveal a similarly anomalous mechanical response in the form of both negative linear compressibility (NLC) and negative Poisson's ratios.