Reaction between [Ru(H2O)6]2+ and DMF: A Complete Volume Profile of a Monocomplex Formation Reaction on a d6 Low Spin Aquaion

Abstract The reaction between [Ru(H2O)6]2+ and dimethylformamide (DMF) was studied as a function of the temperature and the pressure at pH 1. When a slight excess of Ru aquaion is used, [Ru(H2O)5DMF]2+ is the only product of the reaction observed in the NMR spectra and the DMF is bound through the oxygen atom. From variable temperature experiments, the rate constants, activation enthalpies and entropies for the formation and the dissociation of the monocomplex were determined. These are as follows: 104 x kf298=(8.2 ± 0.4) kg mol-1 s-1 ΔΗ☨f= 83.6 ± 3 k J mol-1 Δ S☨f = -23.7 ± 10 J K-1mol-1 for the formation reaction, 103 x kd298 = (1.10 ± 0.04) s-1 ΔΗ☨f= 90.2 ± 2 kJ mol-1 ΔS☨f, = +1.0 ± 7 J Κ-1 mol -1 for the dissociation reaction. The thermodynamic parameters (equilibrium constant at 298.15 K, reaction enthalpy and entropy) were also determined:K298eq = 0.74 ± 0.02 kg mol-1 , ΔH0 = -6.6 ± 2 k j mol-1 , ΔS0 =-24.7 ± 6 JK-1. The kf value is similar to the previously reported rate constants for monocomplex formation reactions with other ligands and confirms that the entering ligand has no effect on the rate. From variable pressure experiments, the activation volumes for the formation and the dissociation reactions and the reaction volume were determined: ΔVf☨ = +2.7 ± 0.6 cm3 mol-1 , ΔVd☨= 0.9 ± 0.7 cm3 mol-1 , ΔV° = +3.6 ± 0 . 9 cm3 mol -1 respectively. These results show that the volume of the transition state is intermediate between the volume of the reactants and the volume of the products. It demonstrates that the mechanism is the same as for the water exchange on [Ru(H2O)6 ] 2+ , i.e. an Id mechanism. Contraction of the bonds between Ru and the spectator ligands at the transition state may explain the abnormally small activation volumes. The need of theoretical calculations on these systems is emphasized.