Cyclooctadiene rhodium chloride dimer
# Cyclooctadiene rhodium chloride dimer

Cyclooctadiene rhodium chloride dimer is the organorhodium compound with the formula $Rh_2Cl_2(C_8H_{12})_2,$ commonly abbreviated $[RhCl(COD)]_2$ or $Rh_2Cl_2(COD)_2.$ This yellow-orange, air-stable compound is a widely used precursor to homogeneous catalysts.

## Preparation and reactions

The synthesis of $[RhCl(COD)]_2$ involves heating a solution of hydrated rhodium trichloride with 1,5-cyclooctadiene in aqueous ethanol in the presence of sodium carbonate:

    2 $RhCl_3·3H_2O$ + 2 COD + 2 $CH_3CH_2OH$ + 2 $Na_2CO_3$ → $[RhCl(COD)]_2$ + 2 $CH_3CHO$ + 8 $H_2O$ + 2 $CO_2$ + 4 NaCl

$[RhCl(COD)]_2$ is principally used as a source of the electrophile $"[Rh(COD)]^+."$

    $[RhCl(COD)]_2$ + nL → $[L_nRh(COD)]^+Cl^−$ (where L = $PR_3,$ alkene, etc. and n = 2 or 3)

In this way, chiral phosphines can be attached to Rh. The resulting chiral complexes are capable of asymmetric hydrogenation. A related but still more reactive complex is chlorobis(cyclooctene)rhodium dimer. The dimer reacts with a variety of Lewis bases (L) to form adducts with the stoichiometry RhCl(L)(COD).

## Structure

The molecule consists of a pair of square planar Rh centers bound to a 1,5-cyclooctadiene and two chloride ligands that are shared between the Rh centers. The $Rh_2Cl_2$ core is also approximately planar, in contrast to the highly bent structure of cyclooctadiene iridium chloride dimer where the dihedral angle is 86°.

