KINETIC STUDY OF CARBONATE CHELATE RING OPENING OF (EN)2COCO3+ IN ALKALINE SOLUTION

A study has been made of the rate of carbonate chelate ring opening and closing in alkaline solution, over the concentration range 1.19 × 10−3 to 0.90 M NaOH and ionic strength (μ) 1.0 M (NaClO1), between 26 and 44°. The results are interpreted in terms of the reactions (en)2CoCO3+ + OH−⇄ (en)2CoOHCO3 k1, k-1 (3) and (en)2CoCO3+ + H2O ⇄ (en)2CoOHCO3H+ k2, k-2 (4). If the system described by (3) and (4) is approaching equilibrium, under conditions in which [OH−] is in large excess and therefore constant, then the observed rate constant kobsd = k1[OH−] + k-1 + k1[H1O] + k-2Kw/K3OH−], where K3 is the acid dissociation constant of (en2)CoOHCO3H+. The individual rate constants were determined by fitting kobsd to this equation. The values for the rate constants (at 26°), activation enthalpies (kcal mole−1), and entropies (eu) are: k1 3.17 × 101 M-1 sec-1, 22.0, 3.6; k-1 6.52 × 10−5 sec−1, 28.2,16.6; k2 [H2O], 5.4 × 10−5 sec−1, 21.4, -6.6; k-2Kw/K3, 1.11 × 10−1M sec-1, 27.4,10.6. The equilibrium constant for the first reaction (K1) was determined spectrophotometrically as 48.5 ± 2.5 at 26°, μ = 1.0 (NaCIO4), with a ΔH° and ΔS° of –6.46 kcal mole−1 and –13.19 eu, respectively. An 18O tracer study has shown that the k1 path proceeds with Co-O bond cleavage. The results are compared to previous studies on the acid hydrolysis and carbonate exchange of (en2)CoCO3+, and to work on phosphate, sulfate, and oxalate chelates. © 1969, American Chemical Society. All rights reserved.