Rhodium(III) chloride

Rhodium(III) chloride usually refers to hydrated rhodium trichloride, a molecular compound with the formula RhCl3(H2O)3. Another prominent rhodium chloride is RhCl3, a polymeric solid with the AlCl3 structure. Inexperienced workers sometimes confuse the two rhodium chlorides, but their behavior is completely different. Most chemistry ascribed to "rhodium trichloride" requires the use of the hydrated form. Some procedures calling for a rhodium chloride imply the use of Na3RhCl6, which is also a molecular, hence reactive, form of Rh(III).

Rhodium(III) chlorides are the products of the separation of rhodium from the other platinum group metals.

Properties
RhCl3(H2O)x exists as dark red diamagnetic crystals. It is mildly hygroscopic. It is soluble in water to give reddish solutions that, depending on the age of the solution, contain varying proportions of RhCl3(H2O)3, [RhCl2(H2O)4]+, and [RhCl(H2O)5]2+.

Preparation
RhCl3(H2O)3 is produced by the action of hydrochloric acid on hydrated rhodium(III) oxide. RhCl3(H2O)3 can be crystallized from a solution in concentrated hydrochloric acid. This method helps to remove nitrogen-containing impurities.

RhCl3 is prepared by reaction of chlorine with rhodium sponge at 200-300°C. The corresponding reaction in molten sodium chloride affords Na3RhCl6.

Coordination complexes
RhCl3(H2O)3 is used to prepare a variety of complexes, as illustrated below. The Rh(III) complexes are generally kinetically inert with octahedral geometry. Rh(I) derivatives tend to be square planar.

Ammines
Ethanol solutions of RhCl3(H2O)3 react with ammonia to give the pentammine chloride [RhCl(NH3)5]2+. Zinc reduction of this cation followed by the addition of sulfate gives the colourless hydride [ [RhH(NH3)5]SO4. Note that complexes of NH3 are generally referred to as "ammines".

Thioethers
Ethanolic solutions of RhCl3(H2O)3 react with dialkyl sulfides.
 * RhCl3(H2O)3 +   3 SR2  →  RhCl3(SR2)3 + 3 H2O

Both fac and mer stereoisomers of such compounds have been isolated.

Tertiary phosphines
Reaction of RhCl3(H2O)3 under mild conditions with tertiary phosphines affords adducts akin to the aforementioned thioether complexes. When these reactions are conducted in boiling ethanol solution, one obtains Rh(I) derivatives such as RhCl(PPh3)3, Wilkinson's catalyst. In this case, ethanol probably serves as the reducing agent, affording acetaldehyde.
 * RhCl3(H2O)3 + 3 PPh3  +  CH3CH2OH  →  RhCl(PPh3)3  +  CH3CHO  +  2 HCl  +  3 H2O

Alternatively, PPh3/H2O could be the reductant, affording OPPh3 and HCl.

Pyridine
Upon boiling in a mixture of ethanol and pyridine (py), RhCl3(H2O)3 gives trans-[RhCl2(py)4)]Cl. The reducing influence of the ethanol is apparent because the corresponding reaction in water affords fac-RhCl3(pyridine)3, analogous to the thioether derivatives. Oxidation of aqueous ethanolic solution of pyridine and RhCl3(H2O)3 by air affords blue paramagnetic [ Cl(py)4Rh-O2Rh(py)4Cl ] 5+.

Alkenes
Reaction of RhCl3(H2O)3 with olefins affords compounds of the type Rh2Cl2(alkene)4. Most commonly, dialkenes are employed in this reaction, such as norbornadiene and 1,5-cyclooctadiene. Illustrative of its high reactivity of its alkene complexes, when 1,3-cyclooctadiene is treated with RhCl3(H2O)3 in ethanol, one obtains the 1,5-cyclooctadiene complex. The diolefin ligands can be removed by decomplexation using cyanide.

Carbon monoxide
Stirring a methanol solution of RhCl3(H2O)3 under 1 bar of carbon monoxide produces the dicarbonyldichlororhodate(I) anion, [RhCl2(CO)2]&minus;. Treatment of solid RhCl3(H2O)3 with flowing CO gives [RhCl(CO)2]2, a red solid which in turn dissolves in alcohols to in the presence of chloride to give the aforementioned dichloride.

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. This same compound can be prepared using formaldehyde in place of CO. Trans-RhCl(CO)(PPh3)2 reacts with a mixture of NaBH4 and PPh3 gives RhH(CO)(PPh3)3.

Rhodium and catalysis
Beginning especially in the 1960's, RhCl3(H2O)3 was demonstrated to be catalytically active for a variety of reactions involving CO, H2, and alkene. For example, RhCl3(H2O)3 was shown to dimerise ethene to a mixture of cis and trans 2-butene:
 * 2 C2H4 →  CH3-CH=CH-CH3

(Unfortunately this reaction fails for higher alkenes).

Over the following decades, however, rhodium-based catalysis has emphasized reactions in organic solvents using organic ligands in place of H2O. Thus, ethylene dimerization was shown to involve catalysis by Rh2Cl2(C2H4)4. This and many related discoveries nurtured the then young field of "homogeneous catalysis", wherein the catalysts are dissolved in the medium with the substrate. Previous to this era, most metal catalysts were "heterogeneous", i.e. the catalysts were solids and the substrates were either liquid or gases.

A significant advance in homogeneous catalysis was the finding that PPh3-derived complexes were particularly active catalytically as well as soluble in organic solvents. Best known of the phosphine-supported catalysts is RhCl(PPh3)3,: which catalyzes the hydrogenation and isomerization of alkenes. The hydroformylation. of alkenes is catalyzed by the related RhH(CO)(PPh3)3. Catalysis by rhodium is so efficient that it has significantly displaced the previous technology based on less expensive cobalt catalysts.

Safety
Rhodium(III) chloride is not listed under Annex I of Directive 67/548/EEC, but is usually classified as harmful, R22: Harmful if swallowed. Some Rh compounds have been investigated as anti-cancer drugs. Rhodium is not an essential element.

It is listed in the inventory of the Toxic Substances Control Act (TSCA).