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
CAS Number	14523-22-9 Y
3D model (JSmol)	Interactive image
ChemSpider	21241281
ECHA InfoCard	100.035.021
EC Number	238-540-6
UNII	CRZ1K8LPMR Y
InChI InChI=1S/4CO.2ClH.2Rh/c4*1-2;;;;/h;;;;2*1H;;/q;;;;;;2*+1/p-2 Key: FGKDXBICTVUSPK-UHFFFAOYSA-L
SMILES [C-]#[O+].[C-]#[O+].[C-]#[O+].[C-]#[O+].[Cl-].[Cl-].[Rh+].[Rh+]
Properties
Chemical formula	C4O4Cl2Rh2
Molar mass	388.76
Appearance	red brown volatile solid
Density	2.708 g/cm−3
Melting point	120–125 °C (248–257 °F; 393–398 K)
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 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]