KINETICS OF WATER PROTON-EXCHANGE OF (H2O)5CRCH2CN2+ IN AQUEOUS ACID

The solvent water proton transverse (R2) and longitudinal (R1) relaxation rates have been studied for solutions of the organochromium(III) Complex (H2O)5Cr(CH2CN)2+ as a function of H+ concentration (0.01-0.22 M) and temperature (5-70-degrees-C). The temperature dependence of R2 is typical of a chemical exchange process which is assigned to exchange of coordinated water protons because of the similarity to the behavior of Cr(OH2)63+ and (H2O)5Cr(N3)2+. The variation of the rate of exchange with [H+] indicates pathways which are first-order (k1) independent (k2) and inverse first order (k3) in [H+]. The rate constants at 25-degrees-C, DELTAH* (kcal mol-1), and DELTAS* (cal mol-1 K) are as follows: k1 = 6.4 x 10(4) M-1 s-1, 3.65 +/- 0.3, -24.4 +/- 0.9; k2 = 4.9 x 10(3) S-1, 8.0 +/- 0.3, -14.9 +/- 0.9; k3 = 30 M s-1, 9.7 +/- 0.8, -19.4 +/- 2.5. The kinetic results are combined with hydrolysis constants to derive further information on protonation-deprotonation kinetics and second hydrolysis constants for aquachromium(III) complexes. The kinetic parameters and general relaxation effects for the organochromium(III) complex are similar to those for Cr(OH2)63+ and (H2O)5Cr(N3)2+.