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 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