KINETICS AND MECHANISM OF SUBSTITUTION-REACTIONS IN CIS-[RUCL(L)-(DPPE)(2)](+) COMPLEXES (L=RCN, DPPE=PH2PCH2CH2PPH2)

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)(2)C6H3CN]. The kinetics of t hese reactions has been studied in acetone and CH2Cl2 solutions by mon itoring the absorbance changes at 425 nm with a stopped-flow instrumen t. All reactions are first order with respect to the concentration of both the starting complex and the incoming ligand, and the second orde r rate constants at 25 degrees C for the reactions with the three nitr iles are similar to each other. The poor discriminating ability of [Ru Cl(dppe)(2)](+) towards the different incoming ligands used and its se lectivity for cis addition can be explained by considering the Y-disto rted trigonal bipyramidal structure of the complex. The values derived for the activation parameters are in all cases compatible with associ ative activation, with small effects due to the solvent and the incomi ng ligand. Octahedral cis-[RuCl(L)(dppe)(2)](+) compounds partially di ssociate 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 o f both the leaving ligand and the nucleophile. The kinetic data for th ese reactions indicate a dissociative mechanism with formation of [RuC l(dppe)(2)](+) as intermediate. When taken together, the activation pa rameters for these substitution reactions and those for ligand additio n to [RuCl(dppe)(2)](+) lead to complete activation profiles for the s ubstitution reactions in the octahedral complexes and to the thermodyn amic parameters for addition of one ligand to the five-co-ordinated co mplex. These parameters indicate that the energy required to dissociat e L from cis-[RuCl(L)(dppe)(2)](+) is higher than the binding energy o f L to [RuCl(dppe)(2)](+), which has been interpreted in terms of a st ructural reorganisation of the intermediate.