Hexadecacarbonylhexarhodium Explained

Hexadecacarbonylhexarhodium is a metal carbonyl cluster with the formula Rh₆(CO)₁₆. It exists as purple-brown crystals that are slightly soluble in dichloromethane and chloroform. It is the principal binary carbonyl of rhodium.

Discovery and synthesis

Rh₆(CO)₁₆ was first prepared by Hieber in 1943 by carbonylation of RhCl₃·3H₂O at 80 - 230 °C and 200 atm carbon monoxide with silver or copper as a halide acceptor. Hieber correctly formulated the compound as a binary carbonyl, but suggested the formula Rh₄(CO)₁₁, i.e., CO/Rh ratio of 2.75.^[1] The correct formula and structure was subsequently established by Dahl et al. using X-ray crystallography. The correct CO/Rh ratio is 2.66.^[2]

Relative to the original preparation, the carbonylation of a mixture of anhydrous rhodium trichloride and iron pentacarbonyl was shown to give good yields of Rh₆(CO)₁₆.^[3] Other compounds of rhodium are also effective precursors such as [(CO)₂Rh(μ-Cl)]₂ and rhodium(II) acetate:^[4]

3 Rh₂(O₂CCH₃)₄ + 22 CO + 6 H₂O → Rh₆(CO)₁₆ +  6 CO₂ + 12 CH₃COOH

3 [(CO)₂RhCl]₂ + 4 CO + 6 Cu → Rh₆(CO)₁₆ + 6 CuCl

It also arises quantitatively by thermal decomposition of tetrarhodium dodecacarbonyl in boiling hexane:

Reactions

At least some of the CO ligands can be displaced by donor ligands.^[5]

Rh₆(CO)₁₆ catalyzes a number of organic reactions including hydrogenation and hydroformylation.^[3]

Notes and References

1. 10.1002/zaac.19432510110. Über Metallcarbonyle. XLV. Das Rhodium im System der Metallcarbonyle. 1943. Hieber. W.. Lagally. H.. Zeitschrift für Anorganische und Allgemeine Chemie. 251. 1. 96–113.
2. Rh₆(CO)₁₆ and its Identity with Previously Reported Rh₄(CO)₁₁ . Eugene R. . Corey . Lawrence F. . Dahl . Lawrence F. Dahl . Wolfgang . Beck . . 1963 . 85 . 8 . 1202–1203 . 10.1021/ja00891a040.
3. 10.1016/S0022-328X(00)87682-2 . Metal carbonyl chemistry IV. The preparation of cobalt and rhodium carbonyls by reductive carbonylation with pentacarbonyliron . 1968 . Booth . B. L. . Else . M. J. . Fields . R. . Goldwhite . H. . Haszeldine . R. N. . . 14 . 2 . 417–422.
4. B. R. . James . G. L. . Rempel . W. K. . Teo . Hexadecacarbonylhexarhodium . . 16 . 1976 . 10.1002/9780470132470.ch15 . 49.
5. 10.1002/9780470132623.ch37. Acetonitrile-Substituted Derivatives of Rh₆(CO)₁₆ : Rh₆(CO)_16-x(NCMe)_x (x = 1,2) . Inorganic Syntheses . 1977 . Tunik . S. P. . Vlasov . A. V. . Krivykh . V. V. . 239–244. 31.