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.

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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-nL_n + 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

1. ^ S. Martinengo, G. Giordano, P. Chini, G. W. Parshall, E. R Wonchoba (1990). "Tri-µ-carbonyl-nonacarbonyltetrarhodium". Inorganic Syntheses 28: 242 – 245. doi:10.1002/9780470132593.ch62.
2. ^ Serp, P.; Kalck, P.; Feurer, R.; Morancho, R. (1998). "Tri-µ-carbonyl-nonacarbonyltetrarhodium". Inorganic Syntheses 32: 284 – 287. doi:10.1002/9780470132630.ch45.
3. ^ Brown, D. T.; Eguchi, T.; Heaton, B. T.; Iggo, J. A.; Whyman, R. (1991). "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: 677 - 683. doi:10.1039/DT9910000677.

General reading

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