Homopolynuclear and heteropolynuclear Rh(III) aqua ions - a review

The kinetic inertness of Rh(III) has facilitated investigations of the early stages of hydrolytic polymerization of [Rh(OH₂)₆]³⁺. These studies have led to the synthesis, solution characterization and determination of the X-ray crystal structure of a series of polynuclear aqua ions, which range in nuclearity from binuclear to tetranuclear. The inertness of Rh(III) has enabled the identification of three structural forms of the trinuclear aqua ion which vary in their degree of condensation as well as detailed kinetic and mechanistic studies of water exchange on the binuclear aqua ion, [Rh₂(μ-OH)₂(OH₂)₈]⁴⁺, applying both ¹⁸O isotopic labeling and ¹⁷O NMR spectroscopy. Strategies have been developed which led to the quite efficient synthesis of heteropolynuclear Rh(III)-Cr(III) aqua ions and a heterobinuclear Rh(III)-Ir(III) aqua ion. Elucidation of the composition, properties and structure of heteropolynuclear aqua ions has been possible by applying techniques that had been instrumental in the development of the chemistry of homometallic aqua ions. Kinetic and thermodynamic studies of the Rh(III)-Cr(III) aqua ions generally revealed thermodynamic and activation parameters that corresponded to those for similar processes occurring at Cr(III), as may have been expected given the greater lability of Cr(III) relative to Rh(III). Inductive effects caused by the Rh(III) centers do, however, influence reactivity as exemplified by the slower than expected rate of cleavage of [CrRh₂(μ-OH)₄(OH₂)₁₀]⁵⁺.