The basic mercury(I) chromate(VI), Hg 6 Cr 2 O 9 ( ¼ 2Hg 2 CrO 4 Á Hg 2 O), has been obtained under hydrothermal conditions (200 1C, 5 days) in the form of orange needles as a by-product from reacting elemental mercury and K 2 Cr 2 O 7 . Hydrothermal treatment of microcrystalline Hg 6 Cr 2 O 9 in demineralised water at 200 1C for 3 days led to crystal growth of red crystals of the basic mercury(I, II) chromate(VI), Hg 6 Cr 2 O 10 ( ¼ 2Hg 2 CrO 4 Á 2HgO). The crystal structures were solved and refined from single crystal X-ray data sets. Hg 6 Cr 2 O 9 : space group
0517; Hg 6 Cr 2 O 10 : space group Pca2 1 , Z ¼ 4, a ¼ 11:4745ð15Þ, b ¼ 9:4359ð12Þ, c ¼ 10:3517ð14Þ Å, 3249 structure factors, 114 parameters, R½F 2 42sðF 2 Þ ¼ 0:0398, wR(F 2 all) ¼ 0.0625. Both crystal structures are made up of an intricate mercury-oxygen network, subdivided into single building blocks [O-Hg-Hg-O] for the mercurous compound, and [O-Hg-Hg-O] and [O-Hg-O] for the mixed-valent compound. Hg 6 Cr 2 O 9 contains three different Hg 2 2+ dumbbells, whereas Hg 6 Cr 2 O 10 contains two different Hg 2
2+ dumbbells and two Hg 2+ cations. The Hg I -Hg I distances are characteristic and range between 2.5031(15) and 2.5286(9) A ˚. All Hg 2 2+ groups exhibit an unsymmetrical oxygen environment. The oxygen coordination of the Hg 2+ cations is nearly linear with two tightly bonded O atoms at distances around 2.07 A ˚. For both structures, the chromate(VI) anions reside in the vacancies of the Hg-O network and deviate only slightly from the ideal tetrahedral geometry with average Cr-O distances of ca. 1.66 A ˚. Upon heating at temperatures above 385 1C, Hg 6 Cr 2 O 9 decomposes in a four-step mechanism with Cr 2 O 3 as the end-product at temperatures above 620 1C.