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Coordination Chemistry of Mercury(II) in Liquid and Aqueous Ammonia Solution and the Crystal Structure of Tetraamminemercury(II) Perchlorate

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journal contribution
posted on 17.03.2008 by Kersti B. Nilsson, Mikhail Maliarik, Ingmar Persson, Andreas Fischer, Ann-Sofi Ullström, Lars Eriksson, Magnus Sandström
The ammonia solvated mercury(II) ion has been structurally characterized in solution by means of EXAFS, 199Hg NMR, and Raman spectroscopy and in solid solvates by combining results from X-ray single crystal and powder diffraction, thermogravimetry, differential scanning calorimetry, EXAFS, and Raman spectroscopy. Crystalline tetraamminemercury(II) perchlorate, [Hg(NH3)4](ClO4)2, precipitates from both liquid ammonia and aqueous ammonia solution, containing tetraamminemercury(II) complexes. The orthorhombic space group (Pnma) imposes Cs symmetry on the tetraamminemercury(II) complexes, which is lost at a phase transition at about 220 K. The Hg−N bond distances are 2.175(14), 2.255(16), and 2 × 2.277(9) Å, with a wide N−Hg−N angle between the two shortest Hg−N bonds, 122.1(7)°, at ambient temperature. A similar distorted tetrahedral coordination geometry is maintained in liquid ammonia and aqueous ammonia solutions with the mean Hg−N bond distances 2.225(12) and 2.226(6) Å, respectively. When heated to 400 K the solid tetraamminemercury(II) perchlorate decomposes to diamminemercury(II) perchlorate, [Hg(NH3)2](ClO4)2, with the mean Hg−N bond distance 2.055(6) Å in a linear N−Hg−N unit. The mercury atoms in the latter compound form a tetrahedral network, connected by perchlorate oxygen atoms, with the closest Hg···Hg distance being 3.420(3) Å. The preferential solvation and coordination changes of the mercury(II) ion in aqueous ammonia, by varying the total NH3:Hg(II) mole ratio from 0 to 130, were followed by 199Hg NMR. Solid [Hg(NH3)4](ClO4)2 precipitates while [Hg(H2O)6]2+ ions remain in solution at mole ratios below 3–4, while at high mole ratios, [Hg(NH3)4]2+ complexes dominate in solution. The principal bands in the vibrational spectrum of the [Hg(NH3)4]2+ complex have been assigned.

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