Ruthenium(III) chloride

Ruthenium(III) chloride is the chemical compound with the formula RuCl₃. "Ruthenium(III) chloride" more commonly refers to the hydrate RuCl₃·xH₂O. Both the anhydrous and hydrated species are dark brown or black solids. The hydrate, with a varying proportion of water of crystallization, often approximating to a trihydrate, is a commonly used starting material in ruthenium chemistry.

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Preparation and properties

The anhydrous forms of ruthenium(III) chloride are well characterized but rarely used. Crystalline material is usually prepared by heating powdered ruthenium metal to 700 °C under a 4:1 mixture of chlorine and carbon monoxide: the product is carried by the gas stream and crystallises upon cooling.^[1] RuCl₃ exists is two crystalline modifications. The black α-form adopts the CrCl₃-type structure with long Ru-Ru contacts of 346 pm. The dark brown metastable β-form crystallizes in a hexagonal cell; this form consists of infinite chains of face-sharing octahedra with Ru-Ru contacts of 283 pm. The β-form is irreversibly converted to the α-form at 450–600 °C.

RuCl₃ vapour decomposes into the elements at high temperatures (as do all compounds!): the enthalpy change at 750 °C (1020 K), Δ_dissH₁₀₂₀ has been estimated as +240 kJ/mol.

Coordination chemistry

As the most commonly available ruthenium compound, RuCl₃·xH₂O is the precursor to many hundreds of chemical compounds. The noteworthy property of ruthenium complexes, chlorides and otherwise, is the existence of more than one oxidation state, several of are kinetically inert. All second and third-row transition metals form exclusively low spin complexes, whereas ruthenium is special in the stability of adjacent oxidation states, especially Ru(II), Ru(III) (as in the parent RuCl₃·xH₂O) and Ru(IV).

Illustrative complexes derived from "ruthenium trichloride"

• RuCl₂(PPh₃)₃, a chocolate-colored, benzene-soluble species, which in turn is also a versatile starting material. It arises approximately as follows:

2RuCl₃·xH₂O + 7 PPh₃ → 2 RuCl₂(PPh₃)₃ + OPPh₃ + 5 H₂O + 2 HCl

• [RuCl₂(C₆H₆)]₂, also chocolate brown, poorly soluble complex of benzene, arising from 1,3-cyclohexadiene as follows:

2 RuCl₃·xH₂O + 2 C₆H₈ → [RuCl₂(C₆H₆)]₂ + 6 H₂O + 2 HCl + H₂

The benzene ligand can be exchanged with other arenes such as hexamethylbenzene.

• Ru(bipy)₃Cl₂, an intensely luminescent salt with a long-lived excited state, arising as follows:

RuCl₃·xH₂O + 3 bipy + 0.5 CH₃CH₂OH → [Ru(bipy)₃]Cl₂ + 3 H₂O + 0.5 CH₃CHO + HCl

This reaction proceeds via the versatile intermediate cis-Ru(bipy)₂Cl₂.

• [RuCl₂(C₅Me₅)]₂, arising as follows:

2 RuCl₃·xH₂O + 2 C₅Me₅H → [RuCl₂(C₅Me₅)]₂ + 6 H₂O + 2 HCl

[RuCl₂(C₅Me₅)]₂ can be further reduced to [RuCl(C₅Me₅)]₄.

• Ru(C₅H₇O₂)₃, a red, benzene-soluble coordination complex arising as follows:

RuCl₃·xH₂O + 3 C₅H₈O₂ → Ru(C₅H₇O₂)₃ + 3 H₂O + 3 HCl

• RuO₄, an orange CCl₄-soluble oxidant with a tetrahedral structure, which is of some interest in organic synthesis.

Several of these compounds were key to two recent Nobel Prizes. Noyori was awarded the Nobel Prize in Chemistry in 2001 for the development of practical asymmetric hydrogenation catalysts based on ruthenium. Grubbs was awarded the Nobel Prize in Chemistry in 2005 for the development of practical alkene metathesis catalysts based on ruthenium alkylidene derivatives.

Carbon monoxide derivatives

RuCl₃(H₂O)_x reacts with carbon monoxide under mild conditions.^[1] In contrast, iron chlorides do not react with CO. CO reduces the red-brown trichloride to yellowish Ru(II) species. Specifically, exposure of an ethanol solution of RuCl₃(H₂O)_x to 1 atm of CO gives, depending on the specific conditions, [Ru₂Cl₄(CO)₄], [Ru₂Cl₄(CO)₄]^2-, and [RuCl₃(CO)₃]^-. Addition of ligands (L) to such solutions gives Ru-Cl-CO-L compounds (L = PR₃). Reduction of these carbonylated solutions with Zn affords the orange triangular cluster [Ru₃(CO)₁₂].

3 RuCl₃·xH₂O + 4.5 Zn + 12 CO (high pressure) → Ru₃(CO)₁₂ + 3 H₂O + 4.5 ZnCl₂

Sources

• Gmelins Handbuch der Anorganischen Chemie

References

1. ^ *Hill, A. F. ""Simple" Ruthenium Carbonyls of Ruthenium: New Avenues from the Hieber Base Reaction", Angewandte Chemie International Editiion, 2000, volume 39, pages 130-134.

1. ^  Remy, H.; Kühn, M. (1924). "Beiträge zur Chemie der Platinmetalle. V. Thermischer Abbau des Ruthentrichlorids und des Ruthendioxyds". Z. Anorg. Chem. 137 (1): 365-388. doi:10.1002/zaac.19241370127.

• Bennett, M. A.; Huang, T. N.; Matheson, T. W. and Smith, A. K., "(η⁶-Hexamethylbenzene)ruthenium Complexes", Inorganic Syntheses, 1982, volume 21, 74-8.
• Ikariya, T.; Murata, K.; Noyori, R. "Bifunctional Transition Metal-Based Molecular Catalysts for Asymmetric Syntheses" Organic Biomolecular Chemistry, 2006, volume 4, 393–406.

Further reading

• Carlsen, P. H. J. et al. (1981). J. Org. Chem. 46:3936. (catalyst for oxidation reactions)
• Gore, E. S. (1983). Platinum Met. Rev. 27:111. (review)
• Cotton, S. A. "Chemistry of Precious Metals," Chapman and Hall (London): 1997. ISBN 0-7514-0413-6