From solutions of TeBr4 in concentrated aqueous HBr red needles of the crystalline hexabromotellurous acid with the stoichiometric composition H2TeBr6 · 8H2O (1) were isolated (space group P21/n, a = 7.662(5), b = 9.796(6), c = 11.987(8) Å, β = 91.35(4)° at 259 Κ). If the HBr:TeBr4 concentration ratio in the system TeBr4/HBr/H2O was kept low, a novel acid with formal composition HTe3Br13 ·4 H2O (2) could be prepared (space group Cm, a = 13.501(5), b = 15.453(6), c = 9.153(3) Å, β = 138.11(2)° at 140 Κ). The X-ray structural analysis of 1 shows the acid to contain, besides regular octahedral [TeBr6]2- ions (Te-Br: 2.671(4)-2.716(4) Å), [Η9Ο4]+ hydroxonium ions (O ··· O: 2.48(3)-2.62(3) Å). The [H9O4]+ units are linked through a hydrogen bond system (O ··· O: 2.80(3)-2.97(4) Å) to form a three-dimen-sional network. The formula has to be written as [H9O4]2[TeBr6]. 2 is the first acid in this class with a trinuclear anion. Its structural formula is [H9O4][Te3Br13]. As a fragment of the Te4Br,6 cube the anion consists of three edge-sharing octahedra with a central triply bridging Br. The mean bond distances are 2.515 Å for terminal Te-Br, 2.944 Å for Te-μ2Br and 3.077 Å for Te—μ3Br. The Ο···Ο distances in the isolated [H9O4]+ hydroxonium ions are 2.45(3)-2.57(2) A. Raman spectra of both acids are reported.

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