KINETICS OF REACTIONS BETWEEN 1,2-DIAMINOCYCLOHEXANETETRAACETATOCOBALTATE(2) ION AND CYANIDE ION

Cyanide ion reacts very rapidly with cobalt(II)-CyDTA to form a 1:1 mixed complex. The formation rate, studied from 5 to 25° by the temperature-jump relaxation technique, is first order in cobalt-CyDTA and first order in cyanide ion with a rate constant of k1 = 1.8 × 106 M-1 sec-1 at 25°. The activation energy, 0.8 kcal/mol, and the fact that k1 is about 5000 times larger than expected from the characteristic water-exchange value of Co(II), suggest that cobalt-CyDTA-cyanide is a seven-coordinate complex. The stability constant of CoCyCN3- at 25° is 39 M-1 as determined by both temperature-jump and spectrophotometric techniques. The rate of complete replacement of CyDTA by cyanide to give Co(CN)53- (and its oxidized products) is slow. The reaction is first order in CoCyCN3- and second order in CN- with a rate constant equal to 9.5 × 10-3 M-2 sec-1. The mechanism for the replacement of CyDTA from cobalt by cyanide ion is the same as that observed for the reactions of cyanide ion with NiEDTA2- and CoEDTA2-, which also involve three cyanide ions. The cyanide ion displacement of CyDTA from cobalt(II) is 1.2 × 105 times slower than the corresponding displacement of EDTA. © 1969, American Chemical Society. All rights reserved.