Rhodium carbonyl chloride

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
CAS Number	14523-22-9^Y
ECHA InfoCard	100.035.021
PubChem CID	24852228
UNII	CRZ1K8LPMR^Y
Properties
Chemical formula	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

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.

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 126.8° 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:

1/2 Rh₂Cl₂(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]

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. PMID 27829895.
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. PMID 29601194.