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Original scientific paper

The Kinetics of Aquation of cis-Bispyridinetetraaquachromium(III) Ion

A. Bakač ; Ruder Boskovic Institute, Zagreb, Croatia, Yugoslavia
M. Orhanović ; Ruder Boskovic Institute, Zagreb, Croatia, Yugoslavia

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page 57-63

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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.


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