Tetrarhodium dodecacarbonyl

Tetrarhodium dodecacarbonyl is the chemical compound with the formula Rh4(CO)12. This dark-red crystalline solid is the smallest stable binary rhodium carbonyl. It is used as a catalyst in organic synthesis.

Structure, synthesis, reactions
The structure of Rh4(CO)12 is described by a tetrahedral array of four Rh atoms with nine terminal CO ligands and three bridging CO ligands. The structure can be expressed as Rh4(CO)9(µ-CO)3.

It is prepared by treatment of an aqueous solution of rhodium trichloride with activated copper metal under an atmosphere of CO.


 * 4 RhCl3(H2O)3 +  8 Cu  + 22 CO   →  Rh4(CO)12  +  2 CO2  +  8 Cu(CO)Cl  +  4 HCl  +  10 H2O

Alternatively, the compound can be prepared by treatment of a methanolic solution of RhCl3(H2O)3 with CO to afford H[RhCl2(CO)2], followed by carbonylation in the presence of sodium citrate.

The cluster undergoes thermal substitution with phosphorus ligands:
 * Rh4(CO)12-n + n L  →  Rh4(CO)12-nLn  +  n CO

Related metal carbonyls
Because of their relevance to hydroformylation catalysis, the metal carbonyls has been systematically studied to a high degree. The instability of Rh2(CO)8 has been a source of curiosity. The analogous binary carbonyl of cobalt, Co2(CO)8, is well known. Solutions of Rh5(CO)12 under high pressures of CO do convert to the dirhodium compound:
 * Rh4(CO)12 +  4 CO   →   2 Rh2(CO)8

The relative instability of Rh2(CO)8 conforms with a general trend: Ru(CO)5 loses CO spontaneously to give Ru3(CO)12.

General reading
King, R. B., "Rhodium: Organometallic Chemistry" Encyclopedia of Inorganic Chemistry 1994, 7, 3494.