Activation Volumes for a Series of Spontaneous, Acidand Base-Catalysed Aquation Reactions of Aniono trans- [Co(MeNH2)(NH3)4X]2,1+ Complexes (X=Cl-, Br-,ΝO3-,SO42-)

Abstract The volumes of activation for the spontaneous, base-, and acid- catalysed path of the hydrolysis reaction of a series of trans-[Co(MeNH2)(NH3)4X](3-n)+ ions (X = Cl-, Br-, (ONO2)- (OSO3)2-) have been determined in order to establish analogies with the dissociative trends found in previous work with the spontaneous hydrolysis of neutral ligands from the same cores. While for the base catalysed path a significant decrease in the activation volume is found on going from the {Co(NH3)5} to the trans- {Co(MeNH2)(NH3)4} inert skeleton (i.e. 9.8, 12.5, 4.0 and 9.1 cm3 mol-1 for the chloro, bromo, nitrato and sulfato derivatives), no significant changes are observed for the same complexes in the spontaneous reaction. The trends are rationalized in terms of the important changes occurring in electrostriction factors for the DCB and Id intimate mechanisms operating and the important increase in the degree of dissociativeness due to the presence of a trans-methylamino ligand. For the acid catalysed path the differences are much more difficult to assess, specially taking into account the limited information available as well as the inherent errors involved in the rate constant determination.