Rhenium trioxide

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Formula = ReO₃
MolarMass = 234.21 g mol⁻¹
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Rhenium trioxide ("rhenium(VI) oxide", ReO₃) is a red solid with a metallic lustre. It is the only stable trioxide of the Group 7 elements (Mn, Tc, Re).

tructure

Rhenium oxide forms crystals with a primitive cubic unit cell, with a lattice parameter of 3.742 Å (374.2 pm). The structure of ReO₃ is similar to the Perovskite structure (ABO₃), without the large A cation at the centre of the unit cell. Each rhenium atom is surrounded by six oxygen atoms, forming a ReO₆ octahedron. These octahedra share corners to form the 3-dimensional structure [S.J. Heyes (2000) [http://www.ncl.ox.ac.uk/icl/heyes/structure_of_solids/Lecture4/Lec4.html#anchor3 "Structures of Simple Inorganic Solids"] , Department of Chemistry, University of Oxford.] .

Upon viewing a unit cell of ReO₃, one can see that it is composed of eight ⅛ fragments of rhenium atoms, and twelve ¼ fragments of oxygen atoms. These numbers reduce to 1 Re and 3 O, hence the formula of ReO₃. The coordination number of Re in this compound is 6 because each rhenium atom has six neighbouring oxygen atoms which bear opposite charge. The coordination number of O in this compound is 2 because each oxygen atom has 2 neighbouring Re atoms of opposite charge.

Properties

ReO₃ is unusual for an oxide because it exhibits very low resistivity. It behaves more like a metal in that its resistivity decreases as its temperature decreases. At 300 K, its resisitivity is 100.0 nΩ·m, whereas at 100 K, this decreases to 6.0 nΩ·m, 17 times less than at 300 K.

Preparation

Rhenium trioxide can be formed by reducing rhenium(VII) oxide with carbon monoxideGreenwood&Earnshaw, p. 1047.] :

:Re₂O₇ + CO → 2ReO₃ + CO₂

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