Rhodium carbonyl chloride
# Rhodium carbonyl chloride

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.

## 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 $RhCl_2$ planes. The metals are nonbonding.

## Synthesis and reactions

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

    2 $RhCl_3(H_2O)_3$ + 6 CO → $Rh_2Cl_2(CO)_4$ + 2 $COCl_2$ + 6 $H_2O.$

The complex reacts with triphenylphosphine 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 $cis-[RhCl_2(CO)_2]^−$

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 $Rh_2Cl_2(CO)_4$ + $:S(CH_2)_4$ → $RhCl(CO)_2:S(CH_2)_4$ ΔH = -31.8 kJ $mol^{−1}$

This enthalpy corresponds to the enthalpy change for a reaction forming one mole of the product, $RhCl(CO)_2:S(CH_2)_4,$ 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 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.

