Kinetics of the aquation of the nitro-bis-dioximine-cobalt(III)-complexes(III).

In aqueous solutions the bis-dioximine-nitro complexes of cobalt - ([Co(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 aquation reactions leading to the liberation of NO2- ions and to the formation of 2 the corresponding aquo-complexes. The kinetics of this reaction was studied by determining the concentration of the free NO2- ions. The Griess - Ilosvay dizazotation reaction was used for this purpose. The aquation reaction has been found to be of the first order. Its rate constant is very much influenced by the pH, since there are several protolytic pre-equilibria and the different molecular species exhibit very different rate constants. The experimentally found apparent rate constants at different pH values (k(exp)) allowed us to derive the rate constants of the individual species, as well as their acidity constants. By performing the measurements at different temperatures, activitation enthalpy and entropy values have been derived. Their pH dependence can be interpreted in terms of the Co-N bond strength and electric charge of both the complex and the leaving group, suggesting that the aquation occurs according to an S-N 1 type mechanism. In acidic media 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 bond besides the sigma type dative bond contains also a retrodative pi bond 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 weaken the pi bond of the leaving group, the activation energy will be lower as compared to the complexes containing a ligand in trans position which can give only sigma bonds. At higher pH values the leaving group is the NO2- anion which forms stronger pi bonds with the Co than the HNO2 does. Furthermore, the electrostatic attraction between the positively 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 makes easier the substitution of NO2- and a decrease in Delta H and increase in the rate constant is observed. The influence of the trans ligand is similar as with the highly protonated complexes containing HNO2.