Tetrarhodium dodecacarbonyl
Template:Chembox new 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.[1]
- 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.[2]
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:[3]
- 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.
References
- ↑ Martinego, S., Giordano, G., Chini, P., "Tri-µ-carbonyl-nonacarbonyltetrarhodium" Inorg. Syn 1990, 28, 242 – 245.
- ↑ Serp, P.; Kalck, P.; Feurer, R.; Morancho, R. “Tri-µ-carbonyl-nonacarbonyltetrarhodium" Inorganic Syntheses 1998, volume 32, pp. 284 – 287.
- ↑ Brown, D. T.; Eguchi, T.; Heaton, B. T.; Iggo, J. A.; Whyman, R. "High-pressure spectroscopic studies of reactions of the clusters [Rh4(CO)12–x{P(OPh)3}x] (x= 1–4) with carbon monoxide or syngas" Journal of the Chemical Society, Dalton Transactions, 1991, 677 - 683. DOI: 10.1039/DT9910000677
General reading
King, R. B., "Rhodium: Organometallic Chemistry" Encyclopedia of Inorganic Chemistry 1994, 7, 3494.