Trirhenium nonachloride

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Trirhenium nonachloride (ReCl3, also written Re3Cl9) is a dark red, hygroscopic solid that is insoluble in ordinary solvents. It is an early example of a cluster compound with metal–metal bonds and is commonly used as a starting material for making other rhenium complexes.

Structure and formula
- The solid consists of Re3Cl12 units that share three chloride bridges with neighboring units to form sheet-like networks.
- Each rhenium center is coordinated by seven ligands: four bridging chlorides, one terminal chloride, and two Re–Re bonds.
- In solution, the compound behaves as a trimer; in acetic acid it tends to form dimers. Hydrates exist, notably Re3Cl9(H2O)3, with partially hydrated forms like Re3Cl9(H2O)4 also used in synthesis.
- Hydrolysis in water converts it to a hydrated oxide: Re2O3·xH2O.

Key properties
- Molar mass: 292.57 g/mol; density about 4.8 g/cm3.
- Boiling point: about 500°C (decomposes); no melting point reported.
- Solubility: insoluble in ordinary solvents; hydrolyzes in water.
- The compound is corrosive and must be handled with appropriate safety precautions.

Preparation and uses
- Trirhenium nonachloride is prepared by thermal decomposition of rhenium pentachloride (ReCl5) or related hexachlororhenic(IV) acids.
- Vacuum sublimation at around 500°C yields a material that is comparatively unreactive; partially hydrated forms are often more useful for synthesis.
- It can also be made by reacting rhenium with sulfuryl chloride, sometimes with aluminum chloride as a catalyst.
- ReCl3/ Re3Cl9 serves as a convenient starting material for making a variety of other rhenium complexes.

History
- Discovered in 1932.