Tetrarhodium dodecacarbonyl is the chemical compound with the formula Rh<sub>4</sub>(CO)<sub>12</sub>. This dark-red crystalline solid is the smallest binary rhodium carbonyl that can be handled as a solid under ambient conditions. It is used as a catalyst in organic synthesis.
According to X-ray crystallography, features a tetrahedral array of four Rh atoms with nine terminal CO ligands and three bridging CO ligands. The structure can be expressed as Rh<sub>4</sub>(CO)<sub>9</sub>(ü-CO)<sub>3</sub>.
is prepared by treatment of an aqueous solution of rhodium trichloride with activated copper metal under an atmosphere of CO.
Alternatively, the compound can be prepared by treatment of a methanolic solution of RhCl<sub>3</sub>(H<sub>2</sub>O)<sub>3</sub> with CO to afford H[RhCl<sub>2</sub>(CO)<sub>2</sub>], followed by carbonylation in the presence of sodium citrate.
The cluster undergoes thermal substitution with phosphine ligands, L:
Tetrarhodium dodecacarbonyl quantitatively decomposes in boiling hexane to afford hexadecacarbonylhexarhodium:
Because of their relevance to hydroformylation catalysis, rhodium carbonyls have been systematically studied to a high degree. The instability of Rh<sub>2</sub>(CO)<sub>8</sub> has been a source of curiosity. The analogous binary carbonyl of cobalt, Co<sub>2</sub>(CO)<sub>8</sub>, is well known. Solutions of Rh<sub>4</sub>(CO)<sub>12</sub> under high pressures of CO convert to the dirhodium compound:
Unlike Co<sub>2</sub>(CO)<sub>8</sub> which features bridging carbonyls, the main isomer of Rh<sub>2</sub>(CO)<sub>8</sub> features only terminal CO ligands. The relative instability of Rh<sub>2</sub>(CO)<sub>8</sub> is analogous to the tendency of Ru(CO)<sub>5</sub> to convert to Ru<sub>3</sub>(CO)<sub>12</sub>.