PREPARATION AND CRYSTAL-STRUCTURE OF HG9AS4O16, A MERCURY ARSENATE(V)WITH TRIANGULAR HG-3(4+) CATIONS

The new compound Hg9As4O16 has been prepared by thermal decomposition of the mercury(I) arsenate(V) (Hg-2)(3)(AsO4)(2) in the presence of Hg O in an evacuated, sealed silica tube. It crystallizes with a rhombohe dral structure (R3c, Z = 6), which has been determined from two single -crystal diffractometer data sets collected at 295 K (a = 1665.3(1) pm , c = 1083.8(1) pm, R = 0.051 for 2353 structure factors and 62 variab le parameters) and 170 K (a = 1660.3(2) pm, c = 1083.0(1) pm, R = 0.03 7 for 1681 F-values). It corresponds to the formula (Hg-3(4+))(3)(AsO4 3-)(4) with almost equilateral mercury triangles and nearly perfect As O4 tetrahedra. The average Hg-Hg bond distance of 267.5 pm is consider ably greater than the average Hg-Hg distance of 251.3 pm found in merc ury(I) compounds, reflecting the intermediate (I/II) valence of the me rcury atoms.