Rhenium trioxide or rhenium(VI) oxide is an inorganic compound with the formula ReO<sub>3</sub>. It is a red solid with a metallic lustre that resembles copper in appearance. It is the only stable trioxide of the Group 7 elements (Mn, Tc, Re).
Rhenium trioxide can be formed by reducing rhenium(VII) oxide with carbon monoxide at 200 ðC or elemental rhenium at 400 ðC.
Re<sub>2</sub>O<sub>7</sub> can also be reduced with dioxane.
Rhenium trioxide crystallizes with a primitive cubic unit cell, with a lattice parameter of 3.742 ÃÂ (374.2 pm). The structure of ReO<sub>3</sub> is similar to that of perovskite (ABO<sub>3</sub>), without the large A cation at the centre of the unit cell. Each rhenium center is surrounded by an octahedron defined by six oxygen centers. These octahedra share corners to form the 3-dimensional structure. The coordination number of O is 2, because each oxygen atom has 2 neighbouring Re atoms.
ReO<sub>3</sub> is unusual for an oxide because it exhibits very low resistivity. It behaves like a metal in that its resistivity decreases as its temperature decreases. At 300 K, its resistivity is 100.0 né÷m, whereas at 100 K, this decreases to 6.0 né÷m, 17 times less than at 300 K.
Rhenium trioxide is insoluble in water, as well as dilute acids and bases. Heating it in base results in disproportionation to give and , while reaction with acid at high temperature affords . In concentrated nitric acid, it yields perrhenic acid. Upon heating to 400 ðC under vacuum, it undergoes disproportionation:
Rhenium trioxide can be chlorinated to give rhenium trioxide chloride:
Rhenium trioxide serves as host for the intercalation of one and two equivalents of lithium. Lithium can be incorporated using butyl lithium:
Re remains octahedral before and after intercalation, but the framework distorts.
Rhenium trioxide finds some use in organic synthesis as a catalyst for amide reduction.