Rhodium(III) chloride

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Rhodium(III) chloride
Other names
Rhodium trichloride
3D model (JSmol)
ECHA InfoCard 100.030.138 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 233-165-4
PubChem CID
RTECS number
  • VI9290000
  • InChI=1S/3ClH.Rh/h3*1H;/q;;;+3/p-3 Yes check.svgY
  • InChI=1S/3ClH.Rh/h3*1H;/q;;;+3/p-3
  • [Rh+3].[Cl-].[Cl-].[Cl-]
Molar mass 209.26 g/mol
Appearancedark red solid
Density 5.38 g/cm3, solid
Melting point ca.450 °C (842 °F; 723 K)
Boiling point 717 °C (1,323 °F; 990 K)
Solubility soluble in hydroxide and cyanide solutions, also soluble in aqua regia
Acidity (pKa)acidic in solution
-7.5·10−6 cm3/mol
Monoclinic, mS16
C12/m1, No. 12
−234 kJ/mol
Flash point Nonflammable
Lethal dose or concentration (LD, LC):
>500 mg/kg (rat, oral)
1302 mg/kg (rat, oral) [1]
Safety data sheet (SDS) ICSC 0746
Related compounds
Other anions
Rhodium(III) fluoride
Rhodium(III) bromide
Rhodium(III) iodide
Other cations
Cobalt(II) chloride
Iridium(III) chloride
Related compounds
Ruthenium(III) chloride
Palladium(II) chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Rhodium(III) chloride refers to inorganic compounds with the formula RhCl3(H2O)n, 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 (n = 3) salt is widely used to prepare compounds used in homogeneous catalysis, notably for the industrial production of acetic acid and hydroformylation. [2]



Rhodium trichloride and its various hydrates can be considered the default halides of rhodium. By contrast, its lighter congener cobalt does not form a stable trichloride, mainly being available as cobalt(II) chloride.

Anhydrous rhodium(III) chloride

Anhydrous rhodium chloride is a dense red-brown solid. According to X-ray crystallography, tt crystallises in the motif seen also for YCl3 and AlCl3 (see image in upper right). The metal centres are octahedral, and the halides are doubly bridging. The octahedral molecular geometry adopted by RhCl3 is characteristic of most rhodium(III) complexes. [3] The anhydrous material is insoluble in common solvents and, for that reason, of little value in the laboratory.

Hydrates and aqueous solutions

Although hydrated rhodium trichloride is widely marketed and often used, the structure of this red solid has not been elucidated crystallographically. This reddish solid (see picture in box) is often described as RhCl3(H2O)3, but this composition has not been confirmed crystallographically.

Aqueous solutions of "rhodium trichloride hydrate" have been characterized by 103Rh NMR spectroscopy. Several species are detected, the proportions of which change with time and depend on the concentration of chloride. The relative distribution of these species determines the colour of the solutions, which can range from yellow (the hexaaquo ion) to "raspberry-red". Some of these species are the aquo complexes [Rh(H2O)6]3+, [RhCl(H2O)5]2+, cis- and trans-[RhCl2(H2O)4]+, and two isomers of [RhCl3(H2O)3]. [4] These species have been separated by ion exchange chromatography and individually characterized by UV-vis spectroscopy. [5]


RhCl3(H2O)3 is produced from salts such as Na3RhCl6, the latter being obtained in the purification of rhodium from the other platinum group metals such as platinum and iridium. The trisodium salt is converted to H3RhCl6 by ion exchange chromatography. Recrystallization of this acidic salt from water affords the hydrated trichloride, sometimes called "soluble rhodium trichloride." [6] Anhydrous RhCl3 is prepared by reaction of chlorine with rhodium sponge metal at 200–300 °C. [7] Above 800 °C, the anhydrous chloride reverts to Rh metal and chlorine. [6]

Coordination complexes

Despite the complexity of its solutions, hydrated rhodium trichloride is the precursor to a wide variety of complexes prepared in high yields.These complexes generally arise by substitution reactions , whereby of water and chloride are replaced by more basic ligands as described in the sections below. These reactions are facilitated by the fact that hydrated rhodium trichloride is soluble in a range of polar organic solvents.

Oxygen and nitrogen-based ligands

Evidence for the affinity of rhodium chlorides for oxygen-based ligands is provided by the chloro-aquo complexes discussed above. Rhodium trichloride reacts with acetylacetone to give rhodium acetylacetonate.

Aqueous solutions of rhodium trichloride react with ammonia to give the salt pentamminerhodium chloride, [RhCl(NH3)5]Cl2. As for other metal-ammine complexes, the term "ammine" refers to ammonia bound to a metal ion as a ligand. Zinc reduction of this cation followed by the addition of sulfate gives the colourless hydride complex [HRh(NH3)5]SO4. [8] Some rhodium ammine chlorides are used in the purification of rhodium from its ores. [9]

Upon boiling in a mixture of ethanol and pyridine (py), hydrated rhodium trichloride converts to trans-[RhCl2(py)4)]Cl. In the absence of a reductant, the reaction affords fac -[RhCl3(py)3], analogous to the thioether derivatives. [3] Oxidation of aqueous ethanolic solution of pyridine and RhCl3(H2O)3 by air affords a blue paramagnetic oxygen-bridged compound, [Cl(py)4Rh-O2-Rh(py)4Cl]5+. [10]

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 rhodiium trichloride in alcohols, which also dissolve the organic ligands. Thus, ethanolic solutions of hydrated rhodium trichloride react with diethyl sulfide:

"RhCl3(H2O)3" + 3 S(C2H5)2 → RhCl3(S(C2H5)2)3 + 3H2O

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. [3]

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, reduction occurs, leading to rhodium(I) derivatives. A famous derivative is [RhCl(PPh3)3] known as Wilkinson's catalyst. Either the ethanol solvent or the phosphine serves as reductant: [11] [12]

"RhCl3(H2O)3" + 3 P(C6H5)3 + CH3CH2OH → RhCl(P(C6H5)3)3 + 3 H2O + 2 HCl + CH3CHO
"RhCl3(H2O)3" + 4 P(C6H5)3 → RhCl(P(C6H5)3)3 + 2 H2O + 2 HCl + OP(C6H5)3

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 Rh2Cl2(alkene)4. Specifically, ethylene gives chlorobis(ethylene)rhodium dimer ([(C2H4)2Rh(μ−Cl)]2). With 1,5-cyclooctadiene, cyclooctadiene rhodium chloride dimer ([(C8H12)2Rh(μ−Cl)]2) is produced. [13]

Structure of the cyclooctadiene rhodium chloride dimer. Cyclooctadiene-rhodium-chloride-dimer-3D-balls.png
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: [14]

2 C5(CH3)5H + 2 "RhCl3(H2O)3" → [(C5(CH3)5)RhCl2]2 + 2 HCl + 6 H2O

A solution of hydrated rhodium trichloride in methanol reacts with carbon monoxide to produce H[RhCl2(CO)2], which contains the dicarbonyldichloridorhodate(I) anion. Further carbonylation in the presence of sodium citrate as a reductant leads to tetrarhodium dodecacarbonyl, Rh4(CO)12, a rhodium(0) cluster compound. [15] Solid RhCl3(H2O)3 reacts with flowing CO gives the volatile compound [(CO)2Rh(μ-Cl)]2. [16]

Numerous Rh-CO-phosphine complexes 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 nucleophilic 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 the hydroformylation of alkenes. [17]


Beginning especially in the 1960s, RhCl3(H2O)3 was demonstrated to be catalytically active for a variety of reactions involving CO, H2, and alkenes. [18] These compounds are fundamental petrochemical feedstocks, so their manipulation can be consequential. For example, RhCl3(H2O)3 was shown to dimerise ethene to a mixture of cis and trans 2-butene:

2 CH2=CH2 → CH3−CH2−CH=CH2

Ethylene dimerization was shown to involve catalysis by the aforementioned ethylene complexes. 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.

Another advance in homogeneous catalysis was the finding that PPh3-derived complexes were active catalytically as well as soluble in organic solvents, [17] The best known such catalyst being Wilkinson's catalyst that catalyzes the hydrogenation and isomerization of alkenes. [18]

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.


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

Related Research Articles

<span class="mw-page-title-main">Chromium(III) chloride</span> Chemical compound

Chromium(III) chloride (also called chromic chloride) describes any of several chemical compounds with the formula CrCl3 · xH2O, where x can be 0, 5, and 6. The anhydrous compound with the formula CrCl3 is a violet solid. The most common form of the trichloride is the dark green hexahydrate, CrCl3 · 6 H2O. Chromium chlorides find use as catalysts and as precursors to dyes for wool.

<span class="mw-page-title-main">Iron(II) chloride</span> Chemical compound

Iron(II) chloride, also known as ferrous chloride, is the chemical compound of formula FeCl2. It is a paramagnetic solid with a high melting point. The compound is white, but typical samples are often off-white. FeCl2 crystallizes from water as the greenish tetrahydrate, which is the form that is most commonly encountered in commerce and the laboratory. There is also a dihydrate. The compound is highly soluble in water, giving pale green solutions.

<span class="mw-page-title-main">Wilkinson's catalyst</span> Chemical compound

Wilkinson's catalyst is the common name for chloridotris(triphenylphosphine)rhodium(I), a coordination complex of rhodium with the formula [RhCl(PPh3)3] (Ph = phenyl). It is a red-brown colored solid that is soluble in hydrocarbon solvents such as benzene, and more so in tetrahydrofuran or chlorinated solvents such as dichloromethane. The compound is widely used as a catalyst for hydrogenation of alkenes. It is named after chemist and Nobel laureate Sir Geoffrey Wilkinson, who first popularized its use.

<span class="mw-page-title-main">Triphenylphosphine</span> Chemical compound

Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the formula P(C6H5)3 and often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists as relatively air stable, colorless crystals at room temperature. It dissolves in non-polar organic solvents such as benzene and diethyl ether.

<span class="mw-page-title-main">Cyclooctadiene rhodium chloride dimer</span> Chemical compound

Cyclooctadiene rhodium chloride dimer is the organorhodium compound with the formula Rh2Cl2(C8H12)2, commonly abbreviated [RhCl(COD)]2 or Rh2Cl2(COD)2. This yellow-orange, air-stable compound is a widely used precursor to homogeneous catalysts.

<span class="mw-page-title-main">Rhodium(II) acetate</span> Chemical compound

Rhodium(II) acetate is the coordination compound with the formula Rh2(AcO)4, where AcO is the acetate ion (CH
). This dark green powder is slightly soluble in polar solvents, including water. It is used as a catalyst for cyclopropanation of alkenes. It is a widely studied example of a transition metal carboxylate complex.

<span class="mw-page-title-main">Potassium tetrachloroplatinate</span> Chemical compound

Potassium tetrachloroplatinate(II) is the chemical compound with the formula K2PtCl4. This reddish orange salt is an important reagent for the preparation of other coordination complexes of platinum. It consists of potassium cations and the square planar dianion PtCl42−. Related salts are also known including Na2PtCl4, which is brown-colored and soluble in alcohols, and quaternary ammonium salts, which are soluble in a broader range of organic solvents.

Martin Arthur Bennett FRS is an Australian inorganic chemist. He gained recognition for studies on the co-ordination chemistry of tertiary phosphines, olefins, and acetylenes, and the relationship of their behaviour to homogeneous catalysis.

<span class="mw-page-title-main">(Cymene)ruthenium dichloride dimer</span> Chemical compound

(Cymene)ruthenium dichloride dimer is the organometallic compound with the formula [(cymene)RuCl2]2. This red-coloured, diamagnetic solid is a reagent in organometallic chemistry and homogeneous catalysis. The complex is structurally similar to (benzene)ruthenium dichloride dimer.

<span class="mw-page-title-main">Organoiridium chemistry</span>

Organoiridium chemistry is the chemistry of organometallic compounds containing an iridium-carbon chemical bond. Organoiridium compounds are relevant to many important processes including olefin hydrogenation and the industrial synthesis of acetic acid. They are also of great academic interest because of the diversity of the reactions and their relevance to the synthesis of fine chemicals.

<span class="mw-page-title-main">Organorhodium chemistry</span>

Organorhodium chemistry is the chemistry of organometallic compounds containing a rhodium-carbon chemical bond, and the study of rhodium and rhodium compounds as catalysts in organic reactions.

<span class="mw-page-title-main">Metal halides</span>

Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. A few metal halides are discrete molecules, such as uranium hexafluoride, but most adopt polymeric structures, such as palladium chloride.

<span class="mw-page-title-main">Metal-phosphine complex</span>

A metal-phosphine complex is a coordination complex containing one or more phosphine ligands. Almost always, the phosphine is an organophosphine of the type R3P (R = alkyl, aryl). Metal phosphine complexes are useful in homogeneous catalysis. Prominent examples of metal phosphine complexes include Wilkinson's catalyst (Rh(PPh3)3Cl), Grubbs' catalyst, and tetrakis(triphenylphosphine)palladium(0).

<span class="mw-page-title-main">Tris(triphenylphosphine)rhodium carbonyl hydride</span> Chemical compound

Carbonyl hydrido tris(triphenylphosphine)rhodium(I) [Carbonyl(hydrido)tris(triphenylphosphane)rhodium(I)] is an organorhodium compound with the formula [RhH(CO)(PPh3)3] (Ph = C6H5). It is a yellow, benzene-soluble solid, which is used industrially for hydroformylation.

<span class="mw-page-title-main">Cyclooctadiene iridium chloride dimer</span> Chemical compound

Cyclooctadiene iridium chloride dimer is an organoiridium compound with the formula [Ir(μ2-Cl)(COD)]2, where COD is the diene 1,5-cyclooctadiene (C8H12). It is an orange-red solid that is soluble in organic solvents. The complex is used as a precursor to other iridium complexes, some of which are used in homogeneous catalysis. The solid is air-stable but its solutions degrade in air.

<span class="mw-page-title-main">Chlorobis(ethylene)rhodium dimer</span> Chemical compound

Chlorobis(ethylene)rhodium dimer is an organorhodium compound with the formula Rh2Cl2(C2H4)4. It is a red-orange solid that is soluble in nonpolar organic solvents. The molecule consists of two bridging chloride ligands and four ethylene ligands. The ethylene ligands are labile and readily displaced even by other alkenes. A variety of homogeneous catalysts have been prepared from this complex.

<span class="mw-page-title-main">Rhodium carbonyl chloride</span> Chemical compound

Rhodium carbonyl chloride is an organorhodium compound with the formula Rh2Cl2(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.

<span class="mw-page-title-main">Pentamethylcyclopentadienyl rhodium dichloride dimer</span> Chemical compound

Pentamethylcyclopentadienyl rhodium dichloride dimer is an organometallic compound with the formula [(C5(CH3)5RhCl2)]2, commonly abbreviated [Cp*RhCl2]2 This dark red air-stable diamagnetic solid is a reagent in organometallic chemistry.

<span class="mw-page-title-main">Hydridotetrakis(triphenylphosphine)rhodium(I)</span> Chemical compound

Hydridotetrakis(triphenylphosphine)rhodium(I) is the coordination complex with the formula HRh[P(C6H5)3]4. It consists of a Rh(I) center complexed to four triphenylphosphine (PPh3) ligands and one hydride. The molecule has idealized C3v symmetry. The compound is a homogeneous catalyst for hydrogenation and related reactions. It is a yellow solid that dissolves in aromatic solvents.

<span class="mw-page-title-main">Dichlorotetrakis(pyridine)rhodium(III) chloride</span> Chemical compound

Dichlorotetrakis(pyridine)rhodium(III) chloride is the chloride salt of the coordination complex with the formula [RhCl2(pyridine)4]+. Various hydrates are known, but all are yellow solids. The tetrahydrate initially crystallizes from water. The tetrahydrate converts to the monohydrate upon vacuum drying at 100 °C.


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