Ruthenium(III) chloride

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 allotropes of RuCl₃ are known. 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, 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.

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

Coordination chemistry of hydrated ruthenium trichloride

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 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·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 or 1,4-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 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.

Some of these compounds were utilized in the research related 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. 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, [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].