Thioethers and tertiary phosphines

Rhodium(III) also forms a range of complexes with soft Lewis bases, such as thioethers, phosphines, and arsines. Such ligands form Rh(III) complexes, but unlike the "hard" N- and O-based ligands, these complexes often can be reduced to Rh(I) derivatives. The reactions are facilitated by the solubility of rhodium trichloride in alcohols, which also dissolve the organic ligands. Thus, ethanolic solutions of hydrated rhodium trichloride react with diethyl sulfide:

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This complex has been used as source of anhydrous rhodium trichloride that is soluble in lipophilic solvents. Both fac and mer stereoisomers of such complexes have been isolated.

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Alkenes and carbon monoxide

Unlike most other air-stable metal salts, hydrated rhodium trichloride reacts under mild conditions (near room temperature, one atmosphere) with carbon monoxide and many olefins. This behavior opens the doors to extensive inventory of organorhodium compounds. Most of these substrates cause reduction of rhodium(III) to rhodium(I). The resulting Rh(I) complexes engage the carbon-based ligands by pi-backbonding.

Reaction of hydrated rhodium trichloride with olefins affords compounds of the type Rh<sub>2</sub>Cl<sub>2</sub>(alkene)<sub>4</sub>. Specifically, ethylene gives chlorobis(ethylene)rhodium dimer (). With 1,5-cyclooctadiene, cyclooctadiene rhodium chloride dimer () is produced.

thumb|left|Structure of the [[cyclooctadiene rhodium chloride dimer.]]

When hydrated rhodium trichloride is treated with cyclopentadienes, organometallic half sandwich compounds can be produced. For example, treating hydrated rhodium trichloride with pentamethylcyclopentadiene in hot methanol leads to the precipitation of solid pentamethylcyclopentadienyl rhodium dichloride dimer:

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A solution of hydrated rhodium trichloride in methanol reacts with carbon monoxide to produce H[RhCl<sub>2</sub>(CO)<sub>2</sub>], which contains the dicarbonyldichloridorhodate(I) anion. Further carbonylation in the presence of sodium citrate as a reductant leads to tetrarhodium dodecacarbonyl, Rh<sub>4</sub>(CO)<sub>12</sub>, a rhodium(0) cluster compound. Solid RhCl<sub>3</sub>(H<sub>2</sub>O)<sub>3</sub> reacts with flowing CO gives the volatile compound [[rhodium carbonyl chloride|[(CO)<sub>2</sub>Rh(μ-Cl)]<sub>2</sub>]].

Numerous Rh-CO-phosphine complexes have been prepared in the course of extensive investigations on hydroformylation catalysis. RhCl(PPh<sub>3</sub>)<sub>3</sub> reacts with CO to give trans-RhCl(CO)(PPh<sub>3</sub>)<sub>2</sub>, stoichiometrically analogous to but less nucleophilic than Vaska's complex. trans-RhCl(CO)(PPh<sub>3</sub>)<sub>2</sub> reacts with a mixture of NaBH<sub>4</sub> and PPh<sub>3</sub> to give HRh(CO)(PPh<sub>3</sub>)<sub>3</sub>, a highly active catalyst for the hydroformylation of alkenes. The best known such catalyst being Wilkinson's catalyst that catalyzes the hydrogenation and isomerization of alkenes.

The hydroformylation of alkenes is catalyzed by the related RhH(CO)(PPh<sub>3</sub>)<sub>3</sub>. Catalysis by rhodium is so efficient that it has significantly displaced the previous technology based on less expensive cobalt catalysts.

References

  • International Chemical Safety Card 0746
  • NIOSH Pocket Guide to Chemical Hazards