a2
The Reactions of Pentacyanocobaltate(11) with Hydrogen Peroxide, Hydroxylamine, and Cyanogen Iodide' P. B. Chock, R. B. K. Dewar, Jack Halpern, and Lai-Yoong Wong Contribution f r o m the Department of Chemistry, Unicersity of Chicago, Chicago, Illinois 60637. Received August 5 , 1968
Abstract: T h e oxidation of pentacyanocobaItate(I1) by several molecules, namely hydrogen peroxide, hydroxylamine, and cyanogen iodide, was examined i n aqueous solution. T h e reaction in each case conforms t o the stoiCO(CN)~X~f CO(CN)~Y~-, where X-Y = HO-OH, HO-NH2, or I-CN. chiometry ~ C O ( C N ) ~ ~X-Y Each of the reactions exhibits second-order kinetics according t o the rate law -d[X-Y]/dt = ~[CO(CN)~~-][X-Y]. The values of k at 25" (M-l sec-l), of AH* (kcal/mole), and of AS* (eu), respectively, were determined t o b e as follows: for HO-OH, 7.4 X lo2, 4.2, -31; for HO-NH2, 5.3 X 10.3, -35; for I-CN, 9.5, 12.7, -12. The results are interpreted i n terms of free-radical mechanisms of the type: CO(CN)~~- X-Y -+CO(CN)~X~Y. (rate determining); CO(CN)~~- Y . CO(CN)~Y 3-. Added I- interferes with t h e reaction between CO(CN)~+and HzOzby reacting with the OH radical intermediate according t o IOH. -+ I . OH-, with subsequent capture of the I atom by CO(CN)~~to form Co(CN)J3-. In the presence of added I- the stoichiometry thus becomes ZCO(CN)~~- H202 I- + CO(CN)~OH~- Co(CN)JaOH-.
-
+
+
+
-f
+
+
+
mong the reactions which pentacyanocobaltate(I1) has been reported to undergo are a number of reactions involving the reductive cleavage of covalent molecules, the general stoichiometry of which conforms to eq I . The molecules which have been shown to react
A
2Co(CN)a3-
+ X-Y
+Co(CN)rX3.- + Co(CN)iY3-
(1)
with C O ( C N ) ~ in ~ - this way include H2,2-6H z 0 , 3Br2,' 12,' HO-OH,S CH3-I, and various other organic halides. B- 1 1 In this paper we describe kinetic studies on several such reactions, namely
+ HO-OH + HO-NHa + H2O
2Co(CN)s32Co(CN)s'-
Co(CN)jOH+ 2C0(CN)s3-
+ I-CN
2C0(CN)2OH3-
(2)
+ Co(CN)5SHs2- + OH+ CO(CN)G'-
(3)
----f
--+ CO(CN)J-
(4)
In each case the results are interpreted in terms of a free-radical mechanism depicted by eq 5 and 6, analogous to that previously proposed for the r e a c t i o n of CO(CN);~-with organic halides. Co(CN)j3- + X-Y --+ Co(Ch')jX'- + Y . (rate determining) Co(CN);'-
+Y.
CO(CN)~Y-~
(5) (6)
Recently we have also showni2 that other low-spin cobalt(I1) complexes, such as pyridinatobis(dimethy1glyoximato)cobalt(II), react similarly, although more slowly than C O ( C N ) ~ ~with - , organic halides. (1) Support of this work through grants from the National Sciencc Foundation and the Advanced Rcscarch Projects Agency is gratcfully acknowledged. (2) N. I
