The aquation of the cis-bis(iminodiacetato)chronlate(III) ion and the ti.cLns(Sac)-bis(meth3.liminodiacetato)chr~~nate(IlI) ion in acidic aqueous medium has been studied spectrophotometrically. The products of the aquation are the iminodiacetatotriaquochromium(II1) cation and the methyliminodiacetatotriaquochromium( 111) cation, respectively. In the reaction involving the cis-bis(imiiiodiacetato)chromate(III) ion, two distinct aquation steps were observed. The first observable step showed the rate lawThe second step gave the rate lawl'alues of k ' , k". and k"' a t 25.0" and ionic strength 0.9 are 7.08 X 10-3 JI-1 sec-1, 1.71 X 10-' sec-1, and 1.28 X M-' sec-1, respectively. The activation parameters for k', k", and k'" are AH+' = 15.3 f 0.4 kcal mol-', ASF = -17.0 f 1.5 cal deg-1 mol-'; AH& = 17.0 i : 0.6 kcal mol-', AS* = -18.7 ik 2.1 cal deg-1 mol-'; and AH* = 24.3 i : 0.4 kcal mol-', AS* = 5.1 i 1.4 cal deg-' mol-', respectively. In the aquation of the tuans(fac)-bis(metliyliminodiacetato)chromate(III) ion, an aquation step was observed with the rate law = (k'[H10+] + k"[H30]2)[complex] d [complex] dt ~ I-alues of k' and k" at 25.0" and ionic strength 0.9 are 1.88 X 10-4 36-1 sec-1 and 5.74 X J k -2 sec-l, respectively. The activation parameters for k' and k" are AH* = 17.0 i 0.3 kcal mol-1, AS* = -18.6 i 1.1 cal deg-l mol-'; and AH+' = 15.6 It 0.2 kcal mol-', AS'? = -21.2 i . 0.7 cal deg-l mol-', respectively. Reaction mechanisms for both systems, which are in agreement with the experimental data, are proposed.
The slow decomposition of manganese(III) ethylenediaminetetraacetate complex was studied by spectrophotometric methods. The observed kinetics were explained by assuming three parallel reactions all of which were first order in the complex concentration when above 2 mmoles/l. The rate law was found to be: decomposition rate), with rate constants h = 0.60 sec."1 M~\ h2 = 1.70 X 10-6 sec.-1, h2 = 2.62 X 10_8sec. ~l M~l, Kz -0.51 X 10-3 M. The products of all the steps were found to be carbon dioxide, formaldehyde, and manganese(II) complex. The previously reported inhibition due to manganese(II) was shown to be due to its effect in complexing free EDTA. The difference in the rates due to terms one and three of the rate law was explained by the protonation of the free EDTA. It was necessary to invoke a steady-state mechanism to explain the variation of the rate with the concentration of the manganese(III) complex.
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