Rhodium carbonyl chloride

Rhodium carbonyl chloride is an organorhodium compound with the formula Rh2Cl2(CO)4. It is a red-brown volatile solid that is soluble in nonpolar organic solvents. It is a precursor to other rhodium carbonyl complexes, some of which are useful in homogeneous catalysis.

Rhodium carbonyl chloride
Names
IUPAC name
Di-μ-chloro-tetracarbonyldirhodium(I)
Other names
Rhodium carbonyl chloride(I), Rhodium(I) carbonyl chloride, Rhodium(I) dicarbonyl chloride dimer, Tetracarbonyldi-μ−chlorodirhodium(I), Dirhodium tetracarbonyl dichloride
Identifiers
UNII
Properties
C4O4Cl2Rh2
Molar mass 388.76
Appearance red brown volatile solid
Density 2.708 g/cm−3
Melting point 120-125 °C
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Structure

The molecule consists of two planar Rh(I) centers linked by two bridging chloride ligands and four CO ligands. X-ray crystallography shows that the two Rh(I) centers are square planar with the dihedral angle of 53° between the two RhCl2 planes. The metals are nonbonding.[1]

Synthesis and reactions

First prepared by Walter Hieber,[2] it is typically prepared by treating hydrated rhodium trichloride with flowing carbon monoxide, according to this idealized redox equation:

2 RhCl3(H2O)3 + 6 CO → Rh2Cl2(CO)4 + 2 COCl2 + 6 H2O.[3]

The complex reacts with triphenylphosphine to give the bis(triphenylphosphine)rhodium carbonyl chloride:

Rh2Cl2(CO)4 + 4 PPh3 → 2 trans-RhCl(CO)(PPh3)2 + 2 CO

With chloride salts, the dichloride anion forms:

Rh2Cl2(CO)4 + 2 Cl− → 2 cis-[RhCl2(CO)2]−

With acetylacetone, rhodium carbonyl chloride reacts to give dicarbonyl(acetylacetonato)rhodium(I).

The dimer reacts with a variety of Lewis bases (:B) to form adducts RhCl(CO)2:B. Its reaction with tetrahydrothiophene and the corresponding enthalpy are:

1/2Rh2Cl2(CO)4 + :S(CH2)4 → RhCl(CO)2:S(CH2)4   ΔH= -31.8 kJ mol−1

This enthalpy corresponds to the enthalpy change for a reaction forming one mole of the product, RhCl(CO)2:S(CH2)4, from the acid dimer. The dissociation energy for Rhodium(I) dicarbonyl chloride dimer, which is an energy contribution prior to reaction with the donor,

Rh2Cl2(CO)4 →2 RhCl(CO)2

has been determined by the ECW model to be 87.1 kJ mol−1

N-heterocyclic carbene (NHC) ligands react with rhodium carbonyl chloride to give monomeric cis-[RhCl(NHC)(CO)2] complexes. The IR spectra of these complexes have been used to estimate the donor strength of NHCs.[4][5]

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References
  1. Walz, Leonhard; Scheer, Peter "Structure of di-μ-chlorobis[dicarbonylrhodium(I)]" Acta Crystallographica Section C 1991, C47, 640-41. doi:10.1107/S0108270190009404
  2. Hieber, W.; Lagally, H. "Über Metallcarbonyle. XLV. Das Rhodium im System der Metallcarbonyle (Metal carbonyls. XLV. Rhodium in the system of metal carbonyls)" Zeitschrift für Anorganische und Allgemeine Chemie 1943, volume 251, pp. 96-113. doi:10.1002/zaac.19432510110
  3. McCleverty, J. A.; Wilkinson, G. "Dichlorotetracarbonyldirhodium (rhodium carbonyl chloride)" Inorganic Syntheses 1966, volume 8, pp. 211-14. doi:10.1002/9780470132463.ch4
  4. Nonnenmacher, Michael; Buck, Dominik M; Kunz, Doris (23 August 2016). "Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes". Beilstein Journal of Organic Chemistry. 12: 1884–1896. doi:10.3762/bjoc.12.178. PMC 5082490.
  5. Huynh, Han Vinh (30 March 2018). "Electronic Properties of N-Heterocyclic Carbenes and Their Experimental Determination". Chemical Reviews. 118 (19): 9457–9492. doi:10.1021/acs.chemrev.8b00067.
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