Ruthenium trichloride

Ruthenium(III) chloride is the chemical compound with the formula RuCl₃. "Ruthenium(III) chloride" more commonly refers to the hydrate RuCl₃·''x''H₂O. 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.

Preparation and properties

Anhydrous ruthenium(III) chloride is usually prepared by heating powdered ruthenium metal with chlorine. In the original synthesis, the chlorination was conducted in the presence of carbon monoxide, the product being carried by the gas stream and crystallising upon cooling. Two polymorphs of RuCl₃ are known. The black α-form adopts the CrCl₃-type structure with long Ru-Ru contacts of 346 pm. This polymorph has honeycomb layers of Ru³⁺ which are surrounded with an octahedral cage of Cl⁻ anions. The ruthenium cations are magnetic residing in a low-spin J~1/2 ground state with net angular momentum L=1. Layers of α-RuCl₃ are stacked on top of each other with weak Van der Waals forces. These can be cleaved to form mono-layers using .

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, similar to the structure of zirconium trichloride. The β-form is irreversibly converted to the α-form at 450–600 °C. The β-form is diamagnetic, whereas α-RuCl₃ is paramagnetic at room temperature.

RuCl₃ vapour decomposes into the elements at high temperatures; the enthalpy change at 750 °C (1020 K), Δ_diss''H''₁₀₂₀ has been estimated as +240 kJ/mol.

Solid state physics

α-RuCl₃ was proposed as a candidate for a Kitaev quantum spin liquid state when neutron scattering revealed an unusual magnetic spectrum, and thermal transport revealed chiral Majorana Fermions when subject to a magnetic field.

Coordination chemistry of hydrated ruthenium trichloride

As the most commonly available ruthenium compound, RuCl₃·''x''H₂O 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 which 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·''x''H₂O) 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·''x''H₂O + 7PPh₃ → 2RuCl₂(PPh₃)₃ + OPPh₃ + 5H₂O + 2HCl
* Diruthenium tetraacetate chloride, a mixed valence polymer, is obtained by reduction of ruthenium trichloride in acetic acid.
* C₆H₆)">benzene.html" ;"title="uCl₂(benzene">C₆H₆)sub>2 arises from 1,3-cyclohexadiene or 1,4-cyclohexadiene as follows:
::2RuCl₃·''x''H₂O + 2C₆H₈ → [RuCl₂(C₆H₆)]₂ + 6H₂O + 2HCl + H₂
* Ru(bipy)₃Cl₂, an intensely luminescent salt with a long-lived excited state, arising as follows:
::2 RuCl₃·''x''H₂O + 6 bipy + CH₃CH₂OH → 2 u(bipy)₃l₂ + 6 H₂O + CH₃CHO + 2 HCl
:This reaction proceeds via the intermediate cis-Ru(bipy)₂Cl₂.
* C₅Me₅)">Cp*.html" ;"title="uCl₂(Cp*">C₅Me₅)sub>2, arising as follows:
::2RuCl₃·''x''H₂O + 2C₅Me₅H → [RuCl₂(C₅Me₅)]₂ + 6H₂O + 2 HCl
:[RuCl₂(C₅Me₅)]₂ can be further reduced to [RuCl(C₅Me₅)]₄.
*Ru(acetylacetonate, C₅H₇O₂)₃ arises as follows:
::RuCl₃·''x''H₂O + 3C₅H₈O₂ → Ru(C₅H₇O₂)₃ + 3H₂O + 3HCl
* Ruthenium tetroxide, RuO₄ is produced by oxidation.

Some of these compounds were utilized in the research related to two Nobel Prizes. Ryōji Noyori was awarded the Nobel Prize in Chemistry in 2001 for the development of practical asymmetric hydrogenation catalysts based on ruthenium. Robert H. 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. 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, u₂Cl₄(CO)₄ u₂Cl₄(CO)₄sup>2−, and uCl₃(CO)₃sup>−. 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.
::3RuCl₃·''x''H₂O + 4.5Zn + 12CO (high pressure) → Ru₃(CO)₁₂ + 3''x''H₂O + 4.5ZnCl₂

Sources

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References

Further reading

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{{Chlorides

Ruthenium(III) compounds
Chlorides
Platinum group halides
Coordination complexes