Rhodium(III) chloride usually refers to
hydrated rhodium trichloride, a molecular compound with the formula RhCl
3(H
2O)
3. Another prominent rhodium chloride is RhCl
3, a polymeric solid with a monoclinic crystal structure, same as in YCl
3 or
AlCl3Aluminium chloride is a compound of aluminium and chlorine. The solid has a low melting and boiling point, and is covalently bonded. It sublimes at 178 °C. Molten AlCl
3 conducts electricity poorly, unlike more ionic halides such as sodium chloride...
. 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 Na
3RhCl
6, which is also a molecular, hence reactive, form of Rh(III).
Rhodium(III) chlorides are the products of the separation of
rhodiumRhodium is a chemical element that is a rare, silvery-white, hard transition metal and a member of the platinum group. Rhodium is found in platinum ores and is used in alloys with platinum and as a catalyst. It is abbreviated to Rh and has atomic number 45...
from the other
platinum group metalsThe platinum group metals sometimes collectively refers to six metallic elements clustered together in the periodic table....
.
Properties
RhCl
3(H
2O)
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 RhCl
3(H
2O)
3, [RhCl
2(H
2O)
4]
+, and [RhCl(H
2O)
5]
2+.
Preparation
RhCl
3(H
2O)
3 is produced by the action of hydrochloric acid on hydrated
rhodium(III) oxideRhodium oxide is the chemical compound with the formula Rh2O3.-Structure:Rh2O3 has been found in two major forms. The hexagonal form has the corundum structure...
. RhCl
3(H
2O)
3 can be
crystallizedRecrystallization may refer to:*Recrystallization *Recrystallization *Recrystallization...
from a solution in concentrated
hydrochloric acidHydrochloric acid is the solution of hydrogen chloride in water. It is a highly corrosive, strong mineral acid and has major industrial uses. It is found naturally in gastric acid....
. This method helps to remove
nitrogenNitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
-containing impurities.
RhCl
3 is prepared by reaction of
chlorineChlorine Chlorine Chlorine ( , from the Greek word 'χλωρóς' (khlôros, meaning 'pale green'), is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17 (formerly VII, VIIa, or VIIb). As the chloride ion, which is part of common salt and...
with rhodium sponge at 200-300°C. The corresponding reaction in molten
sodium chlorideSodium chloride, also known as salt, common salt, table salt, or halite, is an ionic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
affords Na
3RhCl
6.
Coordination complexes
RhCl
3(H
2O)
3 is used to prepare a variety of
complexesIn chemistry, a coordination complex or metal complex, is a structure consisting of a central atom or ion , bonded to a surrounding array of molecules or anions ....
, as illustrated below. The Rh(III) complexes are generally kinetically inert with octahedral geometry. Rh(I) derivatives tend to be square planar.
Amines
Ethanol solutions of RhCl
3(H
2O)
3 react with ammonia to give the pentammine chloride [RhCl(NH
3)
5]
2+.
ZincZinc , also known as 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
sulfateIn inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
gives the colourless hydride
[[RhH(NH
3)
5]SO
4.
Note that complexes of NH
3 are generally referred to as "ammines".
Thioethers
EthanolEthanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug, best known as the type of alcohol found in alcoholic beverages and in modern thermometers. Ethanol is one of the oldest recreational drugs...
ic solutions of RhCl
3(H
2O)
3 react with
dialkyl sulfidesA thioether is a functional group in organic chemistry that has the structure R1-S-R2 as shown on right. Like many other sulfur-containing compounds, volatile thioethers characteristically have foul odors....
.
-
- RhCl3(H2O)3 + 3 SR2
A thioether is a functional group in organic chemistry that has the structure R1-S-R2 as shown on right. Like many other sulfur-containing compounds, volatile thioethers characteristically have foul odors....
→ RhCl3(SR2)3 + 3 H2O
Both
fac and
mer stereoisomers of such compounds have been isolated.
Tertiary phosphines
Reaction of RhCl
3(H
2O)
3 under mild conditions with tertiary phosphines affords adducts akin to the aforementioned thioether complexes. When these reactions are conducted in boiling
ethanolEthanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug, best known as the type of alcohol found in alcoholic beverages and in modern thermometers. Ethanol is one of the oldest recreational drugs...
solution, one obtains Rh(I) derivatives such as RhCl(PPh
3)
3,
Wilkinson's catalystWilkinson's catalyst is the common name for chlorotrisrhodium, a chemical compound with the formula RhCl
3...
. In this case, ethanol probably serves as the reducing agent, affording acetaldehyde.
-
- RhCl3(H2O)3 + 3 PPh3 + CH3CH2OH
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug, best known as the type of alcohol found in alcoholic beverages and in modern thermometers. Ethanol is one of the oldest recreational drugs...
→ RhCl(PPh3)3Wilkinson's catalyst is the common name for chlorotrisrhodium, a chemical compound with the formula RhCl
3...
+ CH3CHOAcetaldehyde is an organic chemical compound with the formula CH3CHO or MeCHO. It is a flammable liquid. Acetaldehyde occurs naturally in ripe fruit, coffee, and bread, and is produced by plants as part of their normal metabolism...
+ 2 HCl + 3 H2O
Alternatively, PPh
3/H
2O could be the reductant, affording OPPh
3 and HCl.
Pyridine
Upon boiling in a mixture of ethanol and
pyridinePyridine is a simple aromatic heterocyclic organic compound with the chemical formula C
5H
5N used as a precursor to agrochemicals and pharmaceuticals, and is also an important solvent and reagent. It is structurally related to benzene, wherein one CH group in the aromatic...
(py), RhCl
3(H
2O)
3 gives trans-[RhCl
2(py)
4)]Cl. The reducing influence of the ethanol is apparent because the corresponding reaction in water affords fac-RhCl
3(pyridine)
3, analogous to the thioether derivatives.
Oxidation of aqueous ethanolic solution of
pyridinePyridine is a simple aromatic heterocyclic organic compound with the chemical formula C
5H
5N used as a precursor to agrochemicals and pharmaceuticals, and is also an important solvent and reagent. It is structurally related to benzene, wherein one CH group in the aromatic...
and RhCl
3(H
2O)
3 by air affords blue paramagnetic
[Cl(py)
4Rh-O
2Rh(py)
4Cl
]5+.
Alkenes
Reaction of RhCl
3(H
2O)
3 with olefins affords compounds of the type Rh
2Cl
2(alkene)
4. Most commonly, dialkenes are employed in this reaction, such as
norbornadieneNorbornadiene is a bicyclic hydrocarbon. The molecule can be envisioned as a 1,4-cyclohexadiene bridged in the para positions by a methylene group. The compound is related to norbornene in which one of the double bonds is saturated...
and 1,5-cyclooctadiene. Illustrative of its high reactivity of its alkene complexes, when 1,3-cyclooctadiene is treated with RhCl
3(H
2O)
3 in ethanol, one obtains the 1,5-cyclooctadiene complex. The diolefin ligands can be removed by decomplexation using cyanide.
Carbon monoxide
Stirring a
methanolMethanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with formula CH3OH . It is toxic: drinking 10 ml will cause blindness, and as little as 100 ml will cause death...
solution of RhCl
3(H
2O)
3 under 1 bar of
carbon monoxideCarbon monoxide is a colorless, odorless and tasteless gas, yet very toxic to humans. It consists of one carbon atom and one oxygen atom, connected by a covalent double bond and a dative covalent bond...
produces the
dicarbonyldichlororhodate(I) anion, [RhCl
2(CO)
2]
−. Treatment of solid RhCl
3(H
2O)
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-PR
3 (R = organic group) compounds have been prepared in the course of extensive investigations on hydroformylation catalysis. RhCl(PPh
3)
3 reacts with CO to give trans-RhCl(CO)(PPh
3)
2, stoichiometrically analogous to but less reactive than
Vaska's complexVaska's complex is the trivial name for the chemical compound trans-chlorocarbonylbisiridium, which has the formula IrCl[P3]2...
. This same compound can be prepared using formaldehyde in place of CO. Trans-RhCl(CO)(PPh
3)
2 reacts with a mixture of NaBH
4 and PPh
3 gives RhH(CO)(PPh
3)
3.
Rhodium and catalysis
Beginning especially in the 1960s, RhCl
3(H
2O)
3 was demonstrated to be catalytically active for a variety of reactions involving CO, H
2, and
alkeneIn organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
.
For example, RhCl
3(H
2O)
3 was shown to dimerise ethene to a mixture of
cis and
trans 2-butene2-Butene is an acyclic alkene with four carbon atoms. It is the simplest alkene to shown cis/trans-isomerism ; that is, it exists as two geometrical isomers cis-2-Butene and trans-2-Butene .It is a petrochemical, produced by the catalytic cracking of crude oil...
:
-
- 2 C2H4 → CH3-CH=CH-CH3
2-Butene is an acyclic alkene with four carbon atoms. It is the simplest alkene to shown cis/trans-isomerism ; that is, it exists as two geometrical isomers cis-2-Butene and trans-2-Butene .It is a petrochemical, produced by the catalytic cracking of crude oil...
(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 H
2O. Thus, ethylene dimerization was shown to involve catalysis by Rh
2Cl
2(C
2H
4)
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
PPh3Triphenylphosphine 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...
-derived complexes were particularly active catalytically as well as soluble in organic solvents. Best known of the phosphine-supported catalysts is
RhCl(PPh3)3Wilkinson's catalyst is the common name for chlorotrisrhodium, a chemical compound with the formula RhCl
3...
,: which catalyzes the
hydrogenationHydrogenation is the chemical reaction that results from the addition of hydrogen . The process is usually employed to reduce or saturate organic compounds. The process typically constitutes the addition of pairs of hydrogen atoms to a molecule. Catalysts are required for the reaction to be...
and isomerization of alkenes. The
hydroformylationHydroformylation, also known as oxo synthesis, 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
alkeneIn 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)(PPh
3)
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/EECThe 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
harmfulHarmful 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 seven albums to date, the first two and last three on independent labels , the third and fourth via BMG...
,
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 ActThe 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).
Rhodium is not an essential element.
External links