Kinetics and mechanism of substitution reactions in cis-[RuCl(L)(dppe)2]+ complexes (L = RCN, dppe = Ph2PCH2CH2PPh2)†

Abstract

The five-co-ordinated [RuCl(dppe)₂]⁺ complex reacted rapidly with nitriles to form the corresponding octahedral cis-[RuCl(L)(dppe)₂]⁺ compounds [L = MeCN, PhCN, or 2,6-(MeO)₂C₆H₃CN]. The kinetics of these reactions has been studied in acetone and CH₂Cl₂ solutions by monitoring the absorbance changes at 425 nm with a stopped-flow instrument. All reactions are first order with respect to the concentration of both the starting complex and the incoming ligand, and the second order rate constants at 25 °C for the reactions with the three nitriles are similar to each other. The poor discriminating ability of [RuCl(dppe)₂]⁺ towards the different incoming ligands used and its selectivity for cis addition can be explained by considering the Y-distorted trigonal bipyramidal structure of the complex. The values derived for the activation parameters are in all cases compatible with associative activation, with small effects due to the solvent and the incoming ligand. Octahedral cis-[RuCl(L)(dppe)₂]⁺ compounds partially dissociate L in solution to give an equilibrium mixture of the starting complex and [RuCl(dppe)₂]⁺ and so the kinetics of substitution of L by L′ in the octahedral complexes has been studied using an excess of both the leaving ligand and the nucleophile. The kinetic data for these reactions indicate a dissociative mechanism with formation of [RuCl(dppe)₂]⁺ as intermediate. When taken together, the activation parameters for these substitution reactions and those for ligand addition to [RuCl(dppe)₂]⁺ lead to complete activation profiles for the substitution reactions in the octahedral complexes and to the thermodynamic parameters for addition of one ligand to the five-co-ordinated complex. These parameters indicate that the energy required to dissociate L from cis-[RuCl(L)(dppe)₂]⁺ is higher than the binding energy of L to [RuCl(dppe)₂]⁺, which has been interpreted in terms of a structural reorganisation of the intermediate.