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.
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