J. Zsako et C. Varhelyi, KINETICS OF THE AQUATION OF THE NITRO-BIS -DIOXIMINE-COBALT(III)-COMPLEXES(III), Magyar kemiai folyoirat, 104(5), 1998, pp. 188-194
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
.