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Rhodium(III) chloride

2007 Schools Wikipedia Selection. Related subjects: Chemical compounds

                   Rhodium(III) chloride
              Image:Rhodium(III) chloride.jpg
                          General
   Other names                Rhodium trichloride
   Molecular formula          RhCl[3]
   Molar mass                 209.26 g/mol (anhydrous)
   Appearance                 dark red solid
   CAS number                 [10049-07-7] (anhydrous)
   EINECS number              233-165-4
                        Properties
   Density and phase          5.38 g/cm^3, solid
   Solubility in water        soluble
   Melting point              450 °C (uncertain)
   Boiling point              717 °C
   Acidity (pK[a])            acidic in solution
   Standard enthalpy
   of formation Δ[f]H°[solid] -234 kJ/mol
                         Structure
   Coordination
   geometry                   octahedral
   Crystal structure          YCl[3]
                        Safety data
   EU classification          not listed
   PEL-TWA ( OSHA)            0.001 mg/m^3 (as Rh)
   IDLH ( NIOSH)              2 mg/m^3 (as Rh)
   Flash point                non-flammable
   RTECS number               VI9290000
                     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 noted otherwise, data are given for
   materials in their standard state (at 25 °C, 100 kPa)
   Infobox disclaimer and references

   The name rhodium(III) chloride usually refers to hydrated rhodium
   trichloride, a molecular compound with the formula RhCl[3](H[2]O)[3] (
   CAS number [20765-98-4]). Another prominent rhodium chloride is
   RhCl[3], a polymeric solid with the AlCl[3] structure. Inexperienced
   workers sometimes confuse the two rhodium chlorides, but their
   behaviour 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[3]RhCl[6], 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

   RhCl[3](H[2]O)[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[2]O)[3], [RhCl[2](H[2]O)[4]]^+, and [RhCl(H[2]O)[5]]^2+.

Preparation

   RhCl[3](H[2]O)[3] is produced by the action of hydrochloric acid on
   hydrated rhodium(III) oxide. RhCl[3](H[2]O)[3] can be crystallized from
   a solution in concentrated hydrochloric acid. This method helps to
   remove nitrogen-containing impurities.

   RhCl[3] is prepared by reaction of chlorine with rhodium sponge at
   200-300°C. The corresponding reaction in molten sodium chloride affords
   Na[3]RhCl[6].

Coordination complexes

   RhCl[3](H[2]O)[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 RhCl[3](H[2]O)[3] react with ammonia to give the
   pentammine chloride [RhCl(NH[3])[5]]^2+. Zinc reduction of this cation
   followed by the addition of sulfate gives the colourless hydride
   [[RhH(NH[3])[5]]SO[4]. Note that complexes of NH[3] are generally
   referred to as "ammines".

Thioethers

   Ethanolic solutions of RhCl[3](H[2]O)[3] react with dialkyl sulfides.

                RhCl[3](H[2]O)[3] + 3 SR[2] → RhCl[3](SR[2])[3] + 3 H[2]O

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

Tertiary phosphines

   Reaction of RhCl[3](H[2]O)[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(PPh[3])[3],
   Wilkinson's catalyst. In this case, ethanol probably serves as the
   reducing agent, affording acetaldehyde.

                RhCl[3](H[2]O)[3] + 3 PPh[3] + CH[3]CH[2]OH →
                RhCl(PPh[3])[3] + CH[3]CHO + 2 HCl + 3 H[2]O

   Alternatively, PPh[3]/H[2]O could be the reductant, affording OPPh[3]
   and HCl.

Pyridine

   Upon boiling in a mixture of ethanol and pyridine (py),
   RhCl[3](H[2]O)[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 pyridine and
   RhCl[3](H[2]O)[3] by air affords blue paramagnetic
   [Cl(py)[4]Rh-O[2]Rh(py)[4]Cl]^5+.

Alkenes

   Reaction of RhCl[3](H[2]O)[3] with olefins affords compounds of the
   type Rh[2]Cl[2](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 RhCl[3](H[2]O)[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 RhCl[3](H[2]O)[3] under 1 bar of carbon
   monoxide produces the dicarbonyldichlororhodate(I) anion,
   [RhCl[2](CO)[2]]^−. Treatment of solid RhCl[3](H[2]O)[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 complex. 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 1960's, RhCl[3](H[2]O)[3] was demonstrated
   to be catalytically active for a variety of reactions involving CO,
   H[2], and alkene. For example, RhCl[3](H[2]O)[3] was shown to dimerise
   ethene to a mixture of cis and trans 2-butene:

                2 C[2]H[4] → CH[3]-CH=CH-CH[3]

   (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[2]O. Thus, ethylene dimerization was shown to involve catalysis by
   Rh[2]Cl[2](C[2]H[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
   PPh[3]-derived complexes were particularly active catalytically as well
   as soluble in organic solvents. Best known of the phosphine-supported
   catalysts is RhCl(PPh[3])[3],:^ which catalyzes the hydrogenation and
   isomerization of alkenes. The hydroformylation. of alkenes 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 is not an essential element, so it can be assumed to be
   unhealthy. 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).
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