Rhodium(III) chloride
Encyclopedia
Rhodium chloride refers to inorganic compound
s 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 salt is widely used to prepare compounds used in homogeneous catalysis
.
Anhydrous rhodium chloride crystallises with a monoclinic crystal structure, same as in YCl3 or AlCl3
. It is a dense brown solid that is insoluble in common solvents and of little value in the laboratory.
such as platinum and iridium. The sodium 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." Anhydrous RhCl3 is prepared by reaction of chlorine
with rhodium sponge at 200-300 °C. Above 800 °C, the anhydrous chloride reverts to Rh metal and chlorine.
, some of which are commercially useful. Complexes of Rh(III) are generally kinetically inert with octahedral geometry. Rh(I) derivatives tend to adopt square planar molecular geometry or be five-coordinate.
reduction of this cation followed by the addition of sulfate
gives the colourless hydride complex [RhH(NH3)5]SO4.
Upon boiling in a mixture of ethanol and pyridine
(py), hydrated rhodium trichloride converts to 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-O2-Rh(py)4Cl] 5+.
ic solutions of hydrated rhodium trichloride react with dialkyl sulfides
.
Both fac and mer stereoisomers of such compounds have been isolated.
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 of Rh occurs, leading to Rh(I) derivatives such as RhCl(PPh3)3, Wilkinson's catalyst
:
RhCl3(H2O)3 reacts with 1 bar of carbon monoxide
to produce the dicarbonyldichlororhodate(I) anion, [RhCl2(CO)2]−. Treatment of solid RhCl3(H2O)3 with flowing CO gives [RhCl(CO)2]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 RhH(CO)(PPh3)3, a highly active catalyst for hydroformylation.
s.
For example, RhCl3(H2O)3 was shown to dimerise ethene to a mixture of cis and trans 2-butene
:
Unfortunately this reaction fails for higher alkenes.
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. Another advance in homogeneous catalysis was the finding that PPh3
-derived complexes were 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 alkene
s 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.
, 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).
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
s 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 salt is widely used to prepare compounds used in homogeneous catalysis
Homogeneous catalysis
In chemistry, homogeneous catalysis is a sequence of reactions that involve a catalyst in the same phase as the reactants. Most commonly, a homogeneous catalyst is codissolved in a solvent with the reactants.-Acid catalysis:...
.
Structures
Hydrated rhodium trichloride usually refers to compound with the approximate formula RhCl3(H2O)3. 103Rh NMR spectroscopy shows that solutions of this material consist of several species, 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 [Rh(H2O)6]3+, [RhCl(H2O)5]2+, cis- and trans-[RhCl2(H2O)4]+, and [RhCl3(H2O)3]. Individual ions have been separated by ion exchange chromatography.Anhydrous rhodium chloride crystallises with a monoclinic crystal structure, same as in YCl3 or AlCl3
Aluminium chloride
Aluminium chloride is the main compound of aluminium and chlorine. It is white, but samples are often contaminated with iron trichloride, giving it a yellow colour. The solid has a low melting and boiling point. It is mainly produced and consumed in the production of aluminium metal, but large...
. It is a dense brown solid that is insoluble in common solvents and of little value in the laboratory.
Preparation
RhCl3(H2O)3 is produced from salts such as Na3RhCl6, the latter being obtained in the purification of rhodium from the other platinum group metalsPlatinum group
The platinum group metals is a term used sometimes to collectively refer to six metallic elements clustered together in the periodic table.These elements are all transition metals, lying in the d-block .The six...
such as platinum and iridium. The sodium salt is converted to H3RhCl6 by ion exchange chromatography
Ion exchange chromatography
Ion-exchange chromatography is a process that allows the separation of ions and polar molecules based on their charge. It can be used for almost any kind of charged molecule including large proteins, small nucleotides and amino acids. The solution to be injected is usually called a sample, and the...
. Recrystallization
Recrystallization (chemistry)
-Chemistry:In chemistry, recrystallization is a procedure for purifying compounds. The most typical situation is that a desired "compound A" is contaminated by a small amount of "impurity B". There are various methods of purification that may be attempted , which includes recrystallization...
of this acidic salt from water affords the hydrated trichloride, sometimes called "soluble rhodium trichloride." Anhydrous RhCl3 is prepared by reaction of chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
with rhodium sponge at 200-300 °C. Above 800 °C, the anhydrous chloride reverts to Rh metal and chlorine.
Coordination complexes
RhCl3(H2O)3 is the precursor to a wide variety of complexesComplex (chemistry)
In chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...
, some of which are commercially useful. Complexes of Rh(III) are generally kinetically inert with octahedral geometry. Rh(I) derivatives tend to adopt square planar molecular geometry or be five-coordinate.
Amines and pyridine
Solutions of RhCl3(H2O)3 react with ammonia to give salts of the pentammine chloride [RhCl(NH3)5]2+. ZincZinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
reduction of this cation followed by the addition of sulfate
Sulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
gives the colourless hydride complex [RhH(NH3)5]SO4.
Upon boiling in a mixture of ethanol and pyridine
Pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
(py), hydrated rhodium trichloride converts to 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
Pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
and RhCl3(H2O)3 by air affords blue paramagnetic
Thioethers and tertiary phosphines
EthanolEthanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
ic solutions of hydrated rhodium trichloride react with dialkyl sulfides
Thioether
A thioether is a functional group in organosulfur chemistry with the connectivity C-S-C as shown on right. Like many other sulfur-containing compounds, volatile thioethers have foul odors. A thioether is similar to an ether except that it contains a sulfur atom in place of the oxygen...
.
-
- RhCl3(H2O)3 + 3 SR2 → RhCl3(SR2)3 + 3 H2O
Both fac and mer stereoisomers of such compounds have been isolated.
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
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
solution, reduction of Rh occurs, leading to Rh(I) derivatives such as RhCl(PPh3)3, Wilkinson's catalyst
Wilkinson's catalyst
Wilkinson's catalyst is the common name for chlorotrisrhodium, a coordination compound with the formula RhCl3 . It is named after the late organometallic chemist and 1973 Nobel Laureate, Sir Geoffrey Wilkinson who popularized its use.-Structure and basic properties:The compound is a square planar,...
:
-
- RhCl3(H2O)3 + 3 PPh3 + CH3CH2OHEthanolEthanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
→ RhCl(PPh3)3Wilkinson's catalystWilkinson's catalyst is the common name for chlorotrisrhodium, a coordination compound with the formula RhCl3 . It is named after the late organometallic chemist and 1973 Nobel Laureate, Sir Geoffrey Wilkinson who popularized its use.-Structure and basic properties:The compound is a square planar,...
+ CH3CHOAcetaldehydeAcetaldehyde is an organic chemical compound with the formula CH3CHO or MeCHO. It is one of the most important aldehydes, occurring widely in nature and being produced on a large scale industrially. Acetaldehyde occurs naturally in coffee, bread, and ripe fruit, and is produced by plants as part...
+ 2 HCl + 3 H2O
- RhCl3(H2O)3 + 3 PPh3 + CH3CH2OH
Alkenes and carbon monoxide
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 reacts with 1 bar of carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
to produce the dicarbonyldichlororhodate(I) anion, [RhCl2(CO)2]−. Treatment of solid RhCl3(H2O)3 with flowing CO gives [RhCl(CO)2]2.
Numerous Rh-CO-PR3 (R = organic group) compounds have been prepared in the course of extensive investigations on hydroformylation
Hydroformylation
Hydroformylation, also known as oxo synthesis or oxo process, is an important industrial process for the production of aldehydes from alkenes. This chemical reaction entails the addition of a formyl group and a hydrogen atom to a carbon-carbon double bond...
catalysis. RhCl(PPh3)3 reacts with CO to give trans-RhCl(CO)(PPh3)2, stoichiometrically analogous to but less reactive than Vaska's complex
Vaska's complex
Vaska's complex is the trivial name for the chemical compound trans-chlorocarbonylbisiridium, which has the formula IrCl[P3]2. This square planar diamagnetic organometallic complex consists of a central iridium atom bound to two mutually trans triphenylphosphine ligands, carbon monoxide, and a...
. Trans-RhCl(CO)(PPh3)2 reacts with a mixture of NaBH4 and PPh3 to give RhH(CO)(PPh3)3, a highly active catalyst for hydroformylation.
Catalysis
Beginning especially in the 1960s, RhCl3(H2O)3 was demonstrated to be catalytically active for a variety of reactions involving CO, H2, and alkeneAlkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s.
For example, RhCl3(H2O)3 was shown to dimerise ethene to a mixture of cis and trans 2-butene
2-Butene
2-Butene is an acyclic alkene with four carbon atoms. It is the simplest alkene exhibiting cis/trans-isomerism ; that is, it exists as two geometrical isomers cis-2-butene , shown at the right, and trans-2-butene , not shown.It is a petrochemical, produced by the catalytic cracking of crude oil...
:
- 2 C2H4 → CH3-CH=CH-CH32-Butene2-Butene is an acyclic alkene with four carbon atoms. It is the simplest alkene exhibiting cis/trans-isomerism ; that is, it exists as two geometrical isomers cis-2-butene , shown at the right, and trans-2-butene , not shown.It is a petrochemical, produced by the catalytic cracking of crude oil...
Unfortunately this reaction fails for higher alkenes.
Ethylene dimerization was shown to involve catalysis by Rh2Cl2(C2H4)4. This and many related discoveries nurtured the then young field of homogeneous catalysis
Homogeneous catalysis
In chemistry, homogeneous catalysis is a sequence of reactions that involve a catalyst in the same phase as the reactants. Most commonly, a homogeneous catalyst is codissolved in a solvent with the reactants.-Acid 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
Triphenylphosphine
Triphenylphosphine is a common organophosphorus compound with the formula P3 - 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...
-derived complexes were active catalytically as well as soluble in organic solvents. Best known of the phosphine-supported catalysts is RhCl(PPh3)3
Wilkinson's catalyst
Wilkinson's catalyst is the common name for chlorotrisrhodium, a coordination compound with the formula RhCl3 . It is named after the late organometallic chemist and 1973 Nobel Laureate, Sir Geoffrey Wilkinson who popularized its use.-Structure and basic properties:The compound is a square planar,...
,: which catalyzes the hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
and isomerization of alkenes. The hydroformylation
Hydroformylation
Hydroformylation, also known as oxo synthesis or oxo process, is an important industrial process for the production of aldehydes from alkenes. This chemical reaction entails the addition of a formyl group and a hydrogen atom to a carbon-carbon double bond...
of alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s 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/EECDirective 67/548/EEC
The Dangerous Substances Directive is one of the main European Union laws concerning chemical safety. It was made under Article 100 of the Treaty of Rome...
, but is usually classified as harmful
Harmful
Harmful is a rock band from Frankfurt, Germany, founded in 1992 and frequently compared to early Helmet and more occasionally to Blackmail. The band has released eight albums to date, the first two and last three on independent labels , the third and fourth via...
, 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
Toxic Substances Control Act
The Toxic Substances Control Act is a United States law, passed by the United States Congress in 1976, that regulates the introduction of new or already existing chemicals. It grandfathered most existing chemicals, in contrast to the Registration, Evaluation and Authorization of Chemicals ...
(TSCA).