Rhenium pentachloride

Rhenium pentachloride is an inorganic compound with the formula. This red-brown solid is paramagnetic.

Structure and preparation

Rhenium pentachloride has a bioctahedral structure and can be described as Cl₄Re₂ReCl₄. The ₂ part of this formula indicates that two chloride ligands are bridging ligands, i.e. they connect to two Re atoms. The Re-Re distance is 3.74 Å. The motif is similar to that seen for tantalum pentachloride.
This compound was first prepared in 1933, a few years after the discovery of rhenium. The preparation involves chlorination of rhenium at temperatures up to 900 °C. The material can be purified by sublimation.
ReCl₅ is one of the most oxidized binary chlorides of Re. It does not undergo further chlorination. ReCl₆ has been prepared from rhenium hexafluoride. Rhenium heptafluoride is known but not the heptachloride.

It degrades in air to a brown liquid.
Although rhenium pentachloride has no commercial applications, it is of historic significance as one of the early catalysts for olefin metathesis. Reduction gives trirhenium nonachloride.
Oxygenation affords the Re oxychloride:
Comproportionation of the penta- and trichloride gives rhenium tetrachloride.