Hydrothermal Single-Crystal Growth in the Systems Ag/Hg/X/O (X = V^V, As^V): Crystal Structures of (Ag₃Hg)VO₄, (Ag₂Hg₂)₃(VO₄)₄, and (Ag₂Hg₂)₂(HgO₂)(AsO₄)₂ with the Unusual Tetrahedral Cluster Cations (Ag₃Hg)³⁺ and (Ag₂Hg₂)⁴⁺ and Crystal Structure of AgHgVO₄

Single crystals of (Ag3Hg)VO4 (I), (Ag2Hg2)3(VO4)4 (II), AgHgVO4 (III), and (Ag2Hg2)2(HgO2)(AsO4)2 (IV) were grown under hydrothermal conditions (250 °C, 5 d) from starting mixtures of elementary mercury, silver nitrate, ammonium vanadate, and disodium hydrogenarsenate, respectively. All crystal structures were determined from X-ray diffraction data, and their chemical compositions were confirmed by electron microprobe analysis. I crystallizes in the tillmannsite structure, whereas II−IV adopt new structure types: (I) I , Z = 2, a = 7.7095(2) Å, c = 4.6714(2) Å, 730 structure factors, 24 parameters, R[F2 > 2σ(F2)] = 0.0365; (II) I 2d, Z = 4, a = 12.6295(13) Å, c = 12.566(3) Å, 1524 structure factors, 55 parameters, R[F2 > 2σ(F2)] = 0.0508; (III) C2, Z = 4, a = 9.9407(18) Å, b = 5.5730(8) Å, c = 7.1210(19) Å, β = 94.561(10)°, 1129 structure factors, 48 parameters, R[F2 > 2σ(F2)] = 0.0358; (IV) P31c, Z = 2, a = 6.0261(9) Å, c = 21.577(4) Å, 1362 structure factors, 52 parameters, R[F2 > 2σ(F2)] = 0.0477. The most striking structural features of I, II, and IV are the formation of tetrahedral cluster cations (Ag3Hg)3+ and (Ag2Hg2)4+, respectively, built of statistically distributed Ag and Hg atoms with a metal−metal distance of about 2.72 Å. The electronic structure of these clusters can formally be considered as two-electron−four-center bonding. The crystal structure of III differs from the protrusive structure types insofar as silver and mercury are located on distinct crystallographic sites without a notable metal−metal interaction >3.55 Å. All crystal structures are completed by tetrahedral oxo anions XO43- (X = VV, AsV) and for IV additionally by a mercurate group, HgO22-.