Kinetics of replacement of bridging ligands in a diruthenium(II,III) cation

Tetrakis-μ-propionatodiruthenium(II,III) cation reacts with oxalate, in acidic aqueous solution (0.10 M LiCF₃SO₃, H⁺] 0.01 M at 29.4°C) in a two-phase process. An initial rapid change results in a small decrease in absorbance. This is ascribed to replacement of one propionate ligand by oxalate. A subsequent slower reaction, of which the rate is proportional to added oxalate concentration (k_bi 1.3 M⁻¹ s⁻¹), corresponds to a relatively large increase of absorbance at 475 nm, the absorbance maximum of the product. The data indicate formation of a bis(μ-oxalato)bis(μ-propionato)diruthenium(II,III) anion with a formation constant K_f > 2 x 10⁴ M⁻¹ and an extinction coefficient of 1.3 x 10³ M⁻¹ cm⁻¹ at 475 nm. A third, much slower, change results in decomposition of that product. The occurrence of replacement of bridging ligands under mild conditions indicates that tris-μ-carboxylatodiruthenium(II,Ill) species (otherwise unknown) function as kinetic intermediates.