7 research outputs found
The Kinetics of Aquation of cis-Bispyridinetetraaquachromium(III) Ion
cis- and trans-Bispyridinetetraaquachromium(III) ions were
prepared and characterized im solution. The aquation of the cisisomer
was studied as a function of acidity and temperature. It
aquates to Cr(H20)5py3+ ion with no observable isomerization path
and the rate law is of the form - d ln [Cr(H20)4(py)23+]/dt =
= (kmo + k0HKa [H+r 1)/(l +Ka [H+i-1). The enthalpy (kcal moi-1)
and entropy (cal moi-1 K-1) of activation are 25.2 and - 4.6 for the
aquation of the cis-Cr(H20)4.(py)23+ ion (k H,o path) and 25.4 and 4.3
for the aquation of the cis-Cr(H20)s(OH)(py)22+ ion (k0H path). The
standard enthalpy and entropy for the acid dissociation reaction of
the cis-Cr(H20)4(py)23+ ion are 12.7 kca1-1 moi-1 and 23.2 cal moi-1 K 1•The corresponding constants extrapolated to 25 °C have the values
k H,o = 2.0 X 10-7 s-1, k 0H = 1.23 X 10-5 s-1, and Ka= 6.0 X 10-5 M.
The hydroxy species is only about 60 times more reactive than
the aqua species, which is consistent with the contribution of an
associative mechan1sm in the aquation process of the latter species
The Kinetics of Aquation of Amminepentaaquachromium(III) Ion. A Comparison with Unsaturated Amine Ligands
The aquation of amminepentaaquachromtwn(III) ion was
studied as a function of acidity and temperature at ~t = 1.0 M
(HCl04 + NaCl04). The rate law is of the form - dln [Cr(H20)s
(NH3) 3+]/dt = k0 + k_1/[W]. The entha1py (kcal mol-1) and entropy
(cal mo1-1 K-1) of activation for the acid-independent path are
27.8 and - 5.3, and for the acid-inverse path 34.8 and 7.0. The
corresponding specific rates extrapolated to 25 °c are 1.8 x 10-9 s-1
and 7.0 X 10-12 M s-1• Cr(H20)5(NH3) 3+ aquates significantly faster
than expected on the basis of the linear correlation between
- log k 0 and pKa of the free ligands established for the analogous
complexes with a series of substituted pyridine ligands. Implications
of this observation are discussed