Crystal Structure and Characterisation of Mercury(II) Dichromate(VI)

Dark-red single crystals of HgCr₂O₇ were grown by reacting HgO and CrO₃ in excess at 200°C for four days. The crystal structure (space group P3₂, Z = 3, a = 7.2389(10), c = 9.461(2) ?, 1363 structure factors, 57 parameters, RF ²>2σ(F ²)] = 0.0369, wR(F ² all) = 0.0693) was determined from a crystal twinned by merohedry according to (110). It consists of nearly linear HgO₂ units ( `(d)]{\bar {d}} (Hg–O) = 2.02 ?) and dichromate units that are linked into infinite chains ‘O₃Cr–O–CrO₃–Hg–O₃Cr–O–CrO₃’ running parallel to the c-axis. Six additional Hg–O contacts between 2.73 and 2.96 ? stabilise the structural arrangement. The dichromate anion exhibits a staggered conformation with a bent Cr–O–Cr bridging angle of 140.7(6)°. Upon heating above 300°C, HgCr₂O₇ decomposes in a two-step mechanism to Cr₂O₃. The title compound was additionally characterised by vibrational spectroscopy.