KINETICS OF THE AQUATION OF THE NITRO-BIS -DIOXIMINE-COBALT(III)-COMPLEXES(III)

In aqueous solutions the bis-dioximine-nitro complexes of cobalt - ([C o(Diox . H)(2)(NO2)(2)](-), -[Co(Diox . H)(2)(NO2)X](-), -[Co(Diox . H )(2)(NO2)L](O); X--halide or pseudohalide; L -H2O, NH3 etc.) undergo a quation reactions leading to the liberation of NO2- ions and to the fo rmation of 2 the corresponding aquo-complexes. The kinetics of this re action was studied by determining the concentration of the free NO2- i ons. The Griess - Ilosvay dizazotation reaction was used for this purp ose. The aquation reaction has been found to be of the first order. It s rate constant is very much influenced by the pH, since there are sev eral protolytic pre-equilibria and the different molecular species exh ibit very different rate constants. The experimentally found apparent rate constants at different pH values (k(exp)) allowed us to derive th e rate constants of the individual species, as well as their acidity c onstants. By performing the measurements at different temperatures, ac tivitation enthalpy and entropy values have been derived. Their pH dep endence can be interpreted in terms of the Co-N bond strength and elec tric charge of both the complex and the leaving group, suggesting that the aquation occurs according to an S-N 1 type mechanism. In acidic m edia the protonation of the co-ordinated NO2 entails the substitution of neutral HNO2 molecules for water, which needs low activation energy values. These are influenced also by the Co-N bond strength. This bon d besides the sigma type dative bond contains also a retrodative pi bo nd due to the shift of 3 d electrons of the Co towards the antibonding pi orbital of HNO2. Since the trans ligands able to give pi bonds wea ken the pi bond of the leaving group, the activation energy will be lo wer as compared to the complexes containing a ligand in trans position which can give only sigma bonds. At higher pH values the leaving grou p is the NO2- anion which forms stronger pi bonds with the Co than the HNO2 does. Furthermore, the electrostatic attraction between the posi tively charged complex ion and the NO2- anion leads to a considerable increase in the activation energy and diminution of the aquation rate. With increasing pH values the stepwise deprotonation of the complex m akes easier the substitution of NO2- and a decrease in Delta H and inc rease in the rate constant is observed. The influence of the trans lig and is similar as with the highly protonated complexes containing HNO2 .