EARLY STAGES OF THE HYDROLYSIS OF CHROMIUM(III) IN AQUEOUS-SOLUTION .12. KINETICS OF CLEAVAGE OF THE TRIMER AND TETRAMER IN ACIDIC SOLUTION

The kinetics of cleavage of the hydrolytic trimer and tetramer of chro mium(III) have been studied in acidic solution and at elevated tempera tures using spectrophotometric methods. For both reactions, the variat ion in absorbance with time was consistent with the presence of two co nsecutive rate-determining processes. The two reactions were concluded to proceed according to: [GRAPHICS] [GRAPHICS] Product analyses and c onsideration of the stability constants for these oligomers indicated that their overall cleavage reactions were irreversible under the cond itions studied. However, it was unclear whether the initial step in te tramer cleavage was irreversible or not. The dimer was identified as a n intermediate in the cleavage of the trimer. Each of the bridge-cleav age processes (k(1) and k(2)) occurred via acid-dependent pathways, in volving protonation processes in addition to bridge-cleavage processes , while only some of them displayed acid independent pathways. The aci d dependence of the rate constants and associated parameters at 298 K are: (i) for trimer cleavage k(1) = k(b) [H+], where k(b) = 7.96 x 10( -8) M(-1) s(-1) [Delta H = 138(+/-9) kJ mol(-1); Delta S* = 82(+/-25) J K-1 mol(-1), and k(2) = k(c) + k(d)[H+], where k(c) = 1.13 x 10(-8) s(-1) [Delta H = 125(+/-11) kJ mol(-1); Delta S* = 23(+/-32) J K-1 m ol(-1)], and k(d) = 8.80 x 10(-7) M(-1) s(-1) [Delta H = 77(+/-4) kJ mol(-1); Delta S = -104(+/-11) J K-1 mol(-1)]; (ii) for tetramer clea vage k(1) = k(a) + k(b)[H+], where k(a) = 2.87 x 10(-5) s(-1) [Delta H = 114(+/-16) kJ mol(-1); Delta S* = 52(+/-51) J K-1 mol(-1)] and k(b ) = 1.18 x 10(-4) M(-1) s(-1) [Delta H = 76(+/-9) kJ mol(-1); Delta S = -64(+/-31) J K-1 mol(-1)], and k(2) = k(d)[H+], where k(d) = 3.04 x 10(-5) M(-1) s(-1) [Delta H = 85(+/-3) kJ mol(-1); Delta S* = -45(/-8) J K-1 mol(-1)]. For the trimer, cleavage of the dimer to monomer was found to be the rate-determining step at higher temperatures (simi lar to 350 K), while cleavage of the trimer to dimer and monomer becom es rate determining at lower temperatures (similar to 300 K). This cha nge in the rate-determining step is attributable to the much higher De lta H value for k(1) compared with k(2). Possible mechanisms for trim er and tetramer cleavage are discussed and their rates compared to pre viously studied cleavage reactions of Cr-III oligomers. Copyright (C) 1996 Published by Elsevier Science Ltd