page_title: Rhodium(III) chloride
section_title: Alkenes and carbon monoxide
hierarchical_section_title: Rhodium(III) chloride / Coordination complexes / Alkenes and carbon monoxide
caption_attribution_description: Ball-and-stick model of the cyclooctadiene rhodium chloride dimer
context_page_description: Rhodium(III) chloride refers to inorganic compounds with the formula RhCl₃ₙ, where n varies from 0 to 3. These are diamagnetic solids featuring octahedral Rh(III) centres. Depending on the value of n, the material is either a dense brown solid or a soluble reddish salt. The soluble trihydrated salt is widely used to prepare compounds used in homogeneous catalysis, notably for the industrial production of acetic acid and hydroformylation.
context_section_description: Reaction of RhCl₃(H₂O)₃ with olefins affords compounds of the type Rh₂Cl₂(alkene)₄. With 1,5-cyclooctadiene, RhCl₃(H₂O)₃ react in ethanol to give cyclooctadiene rhodium chloride dimer.
  RhCl₃(H₂O)₃ in methanol reacts with carbon monoxide (1 bar) to produce H[RhCl₂(CO)₂], which contains the dicarbonyldichloridorhodate(I) anion; further carbonylation in the presence of sodium citrate leads to the formation of tetrarhodium dodecacarbonyl, Rh₄(CO)₁₂, a rhodium(0) cluster compound.  Treatment of solid RhCl₃(H₂O)₃ with flowing CO gives the dimeric rhodium(I) compoundrhodium carbonyl chloride, [(CO)₂Rh(μ-Cl)]₂.
  Numerous Rh-CO-PR₃ (R = organic group) compounds have been prepared in the course of extensive investigations on hydroformylation catalysis. RhCl(PPh₃)₃ reacts with CO to give trans-RhCl(CO)(PPh₃)₂, stoichiometrically analogous to but less reactive than Vaska's complex. Trans-RhCl(CO)(PPh₃)₂ reacts with a mixture of NaBH₄ and PPh₃ to give HRh(CO)(PPh₃)₃, a highly active catalyst for hydroformylation of alkenes.
  When treated with cyclopentadienes or its derivatives, organometallic half sandwich compounds can be produced.  For example, reacting the trihydrate with pentamethylcyclopentadiene (Cp*H) in hot methanol leads to the precipitation of the pentamethylcyclopentadienyl rhodium dichloride dimer, [Cp*RhCl₂]₂:
  2 Cp*H   +   2 RhCl₃(H₂O)₃      [Cp*RhCl₂]₂   +   2 HCl   +   6 H₂O
  This compound was first prepared from hexamethyl Dewar benzene and RhCl₃(H₂O)₃.  The hydrohalic acid necessary for the ring-contraction rearrangement is generated in situ in methanolic solutions of the rhodium salt, and the second step has been carried out separately, confirming this mechanistic description.  The reaction occurs with the formation of 1,1-dimethoxyethane, CH₃CH(OCH₃)₂, and hexamethylbenzene is produce by a side reaction.  This rhodium(III) dimer can be reduced with zinc in the presence of CO to produce the rhodium(I) complex [Cp*Rh(CO)₂].
caption: Structure of the cyclooctadiene rhodium chloride dimer
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