Rhodium carbonyl chloride is an
organorhodium compound with the formula Rh
2Cl
2(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
In chemistry, homogeneous catalysis is catalysis where the catalyst is in same phase as reactants, principally by a soluble catalyst in a solution. In contrast, heterogeneous catalysis describes processes where the catalysts and substrate are in d ...
.
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 RhCl
2 planes. The metals are nonbonding.
Synthesis and reactions
First prepared by
Walter Hieber
Walter Hieber (18 December 1895 – 29 November 1976) was an inorganic chemist, known as the father of metal carbonyl chemistry. He was born 18 December 1895 and died 29 November 1976. Hieber's father was Johannes Hieber, an influential evange ...
, it is typically prepared by treating
hydrated
In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...
rhodium trichloride
Rhodium is a chemical element; it has symbol Rh and atomic number 45. It is a very rare, silvery-white, hard, corrosion-resistant transition metal. It is a noble metal and a member of the platinum group. It has only one naturally occurring isoto ...
with flowing
carbon monoxide
Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
, according to this idealized redox equation:
:2 RhCl
3(H
2O)
3 + 6 CO → Rh
2Cl
2(CO)
4 + 2 COCl
2 + 6 H
2O.
[McCleverty, J. A.; Wilkinson, G. "Dichlorotetracarbonyldirhodium (rhodium carbonyl chloride)" Inorganic Syntheses 1966, volume 8, pp. 211-14. ]
The complex reacts with
triphenylphosphine
Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the formula P(C6H5)3 and often abbreviated to P Ph3 or Ph3P. It is versatile compound that is widely used as a reagent in organic synthesis and as a l ...
to give the
bis(triphenylphosphine)rhodium carbonyl chloride:
:Rh
2Cl
2(CO)
4 + 4 PPh
3 → 2 ''trans''-RhCl(CO)(PPh
3)
2 + 2 CO
With chloride salts, the dichloride anion forms:
:Rh
2Cl
2(CO)
4 + 2 Cl
− → 2 ''cis''-
hCl2(CO)2sup>−
With
acetylacetone
Acetylacetone is an organic compound with the chemical formula . It is classified as a 1,3-diketone. It exists in equilibrium with a tautomer . The mixture is a colorless liquid. These tautomers interconvert so rapidly under most conditions that ...
, 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
Tetrahydrothiophene is an organosulfur compound with the formula (CH2)4S. The molecule consists of a five-membered saturated ring with four methylene groups and a sulfur atom. It is the saturated analog of thiophene and is therefore the sulf ...
and the corresponding enthalpy are:
:1/2 Rh
2Cl
2(CO)
4 + :S(CH
2)
4 → RhCl(CO)
2:S(CH
2)
4 Δ''H'' = -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(CH
2)
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,
:Rh
2Cl
2(CO)
4 → 2 RhCl(CO)
2
has been determined by the
ECW model
In chemistry, the ECW model is a semi-quantitative model that describes and predicts the strength of Lewis acid–Lewis base interactions. Many chemical reactions can be described as acid–base reactions, so models for such interactions are of pot ...
to be 87.1 kJ mol
−1
N-heterocyclic carbene
A persistent carbene (also known as stable carbene) is an organic molecule whose natural resonance structure has a carbon atom with octet rule, incomplete octet (a carbene), but does not exhibit the tremendous instability typically associated with ...
(NHC) ligands react with rhodium carbonyl chloride to give monomeric cis-
hCl(NHC)(CO)2complexes. The IR spectra of these complexes have been used to estimate the donor strength of NHCs.
References
{{Rhodium compounds
Organorhodium compounds
Homogeneous catalysis
Dimers (chemistry)
Carbonyl complexes
Chloro complexes
Rhodium(I) compounds