Substitution of methanol-d₄ for the coordinated water in the trinuclear complexes, [M₃(μ₃-O)(μ-CH₃COO)₆(H₂O)₃]⁺ (M₃=Ru₃, Rh₃ or Ru₂Rh) in methanol-d₄

The ¹H NMR spectra of acetate methyl signals of the titled three complexes in CD₃OD change with time due to successive substitution of CD₃OD for the coordinated water molecules. The first-order rate constants for the first methanol-d₄ substitution of the triruthenium(III) and trirhodium(III) complexes are 7.7×10^-4 s^-1 (298.2 K) (ΔH‡,=103±6 kJ mol^-1 and ΔS‡=+41±12 J K^-1 mol^-1 at 0-21 °C) and 1.3×10^-3 s^-1 (298.2 K) (ΔH‡=102±9 kJ mol^-1 and ΔS‡=+42±32 J K^-1 mol^-1 at 0-10 °C) per one metal ion, respectively, which are greater by approximately 2 and 6 orders of magnitude, respectively, than the water exchange reactions of the hexaaqua complexes of these metal ions. The trans effect of the central oxide ion is considered as a major factor responsible for the labilization. The first-order rate constants for the mixed-metal rhodium-diruthenium complex at 298.2 K are 9.9×10^-5 (ΔH‡=109±4 kJ mol^-1 and ΔS‡=+44±9 J K^-1 mol^-1) and 7.9×10^-5 s^-1 (ΔH‡=103±3 kJ mol^-1 and ΔS‡=+22±6 J K^-1 mol^-1 at 10.1-35.3 °C) at ruthenium and rhodium centers, respectively, which are c. 10 times smaller than the corresponding values for the homonuclear complexes. The slower rates in the mixed-metal complex indicate that electronic configuration in the molecular orbital based on (metal-dπ)-(oxygen-pπ) interactions plays some role in controlling the substitution rate. On the basis of the activation parameters, a dissociative mechanism is proposed for all these reactions.