3sample.jpg) Trihydrate | |
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| Names | |
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| Other names Rhodium trichloride | |
| Identifiers | |
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3D model (JSmol) | |
| ChemSpider |
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| ECHA InfoCard | 100.030.138 |
| EC Number |
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| Properties | |
| RhCl3 | |
| Molar mass | 209.26 g/mol |
| Appearance | red-brown solid |
| Density | 5.38 g/cm3 |
| insoluble | |
| Solubility | soluble inhydroxide andcyanide solutions, also soluble inaqua regia |
| −7.5·10−6 cm3/mol | |
| Structure | |
| Monoclinic,mS16 | |
| C12/m1, No. 12 | |
| octahedral | |
| Thermochemistry | |
Std enthalpy of formation(ΔfH⦵298) | −234 kJ/mol |
| Hazards | |
| Flash point | Nonflammable |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose) | >500 mg/kg (rat, oral) 1302 mg/kg (rat, oral)[1] |
| Safety data sheet (SDS) | ICSC 0746 |
| Related compounds | |
Otheranions | Rhodium(III) fluoride Rhodium(III) bromide Rhodium(III) iodide |
Othercations | Cobalt(II) chloride Iridium(III) chloride |
Related compounds | Ruthenium(III) chloride Palladium(II) chloride |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Rhodium(III) chloride refers toinorganic compounds with the formula RhCl3(H2O)n, wheren varies from 0 to 3. These are diamagnetic red-brown solids. The soluble trihydrated (n = 3) salt is the usual compound of commerce. It is widely used to prepare compounds used inhomogeneous catalysis.[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 ascobalt(II) chloride.
Anhydrous rhodium chloride is a dense red-brown solid. According toX-ray crystallography, it crystallises in the motif seen also forYCl3 andAlCl3 (see image in upper right). The metal centres are octahedral, and the halides are doubly bridging. Theoctahedral 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.
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 by103RhNMR 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 theaquo complexes [Rh(H2O)6]3+, [RhCl(H2O)5]2+,cis- andtrans-[RhCl2(H2O)4]+, and two isomers of [RhCl3(H2O)3].[4] These species have been separated byion exchange chromatography and individually characterized byUV-vis spectroscopy.[5]
RhCl3(H2O)3 is produced from salts such as Na3RhCl6, the latter being obtained in the purification of rhodium from the otherplatinum group metals such as platinum and iridium. The trisodium salt is converted to H3RhCl6 byion 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 ofchlorine with rhodiumsponge metal at 200–300 °C.[7] Above 800 °C, the anhydrous chloride reverts to Rh metal and chlorine.[6]
Despite the complexity of its solutions, hydrated rhodium trichloride is the precursor to a wide variety ofcomplexes prepared in high yields.These complexes generally arise bysubstitution 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.
Evidence for the affinity of rhodium chlorides for oxygen-based ligands is provided by thechloro-aquo complexes discussed above. Rhodium trichloride reacts with acetylacetone to giverhodium acetylacetonate.
Aqueous solutions of rhodium trichloride react with ammonia to give the saltpentamminerhodium chloride, [RhCl(NH3)5]Cl2. As for othermetal-ammine complexes, the term "ammine" refers toammonia bound to a metal ion as a ligand.Zinc reduction of this cation followed by the addition ofsulfate gives the colourlesshydride 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 andpyridine (py), hydrated rhodium trichloride converts totrans-[RhCl2(py)4)]Cl. In the absence of a reductant, the reaction affordsfac-[RhCl3(py)3], analogous to the thioether derivatives.[3] Oxidation of aqueous ethanolic solution of pyridine and RhCl3(H2O)3 by air affords a blueparamagnetic oxygen-bridged compound,[Cl(py)4Rh-O2-Rh(py)4Cl]5+.[10]
Rhodium(III) also forms a range of complexes withsoft Lewis bases, such asthioethers,phosphines, andarsines. 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 withdiethyl sulfide:[11]
This complex has been used as source of anhydrous rhodium trichloride that is soluble in lipophilic solvents. Bothfac andmerstereoisomers of such complexes have been isolated.[3]
Reaction of RhCl3(H2O)3 under mild conditions withtertiary 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 asWilkinson's catalyst. Either the ethanol solvent or the phosphine serves as reductant:[12][13]
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 oforganorhodium compounds. Most of these substrates cause reduction of rhodium(III) to rhodium(I). The resulting Rh(I) complexes engage the carbon-based ligands bypi-backbonding.
Reaction of hydrated rhodium trichloride with olefins affords compounds of the type Rh2Cl2(alkene)4. Specifically, ethylene giveschlorobis(ethylene)rhodium dimer ([(C2H4)2Rh(μ−Cl)]2). With1,5-cyclooctadiene,cyclooctadiene rhodium chloride dimer ([(C8H12)2Rh(μ−Cl)]2) is produced.[14]
When hydrated rhodium trichloride is treated withcyclopentadienes,organometallichalf sandwich compounds can be produced. For example, treating hydrated rhodium trichloride withpentamethylcyclopentadiene in hot methanol leads to the precipitation of solidpentamethylcyclopentadienyl rhodium dichloride dimer:[15]
A solution of hydrated rhodium trichloride inmethanol reacts withcarbon monoxide to produce H[RhCl2(CO)2], which contains the dicarbonyldichloridorhodate(I) anion. Further carbonylation in the presence ofsodium citrate as a reductant leads totetrarhodium dodecacarbonyl, Rh4(CO)12, a rhodium(0)cluster compound.[16] Solid RhCl3(H2O)3 reacts with flowing CO gives the volatile compound[(CO)2Rh(μ-Cl)]2.[17]
Numerous Rh-CO-phosphine complexes have been prepared in the course of extensive investigations onhydroformylation catalysis. RhCl(PPh3)3 reacts with CO to givetrans-RhCl(CO)(PPh3)2, stoichiometrically analogous to but less nucleophilic thanVaska's complex.trans-RhCl(CO)(PPh3)2 reacts with a mixture of NaBH4 and PPh3 to giveHRh(CO)(PPh3)3, a highly active catalyst for thehydroformylation of alkenes.[18]
Beginning especially in the 1960s, RhCl3(H2O)3 was demonstrated to be catalytically active for a variety of reactions involving CO, H2, andalkenes.[19] These compounds are fundamentalpetrochemical feedstocks, so their manipulation can be consequential. For example, RhCl3(H2O)3 was shown to dimeriseethylene to a mixture ofcis andtrans2-butene:
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 thatPPh3-derived complexes were active catalytically as well as soluble in organic solvents,[18] The best known such catalyst being Wilkinson's catalyst that catalyzes thehydrogenation and isomerization of alkenes.[19]
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 ofDirective 67/548/EEC, but is usually classified asharmful,R22:Harmful if swallowed. Some Rh compounds have been investigated asanti-cancer drugs. It is listed in the inventory of theToxic Substances Control Act (TSCA).
