4211
Inorg. Chem. 1988, 27, 4277-4280
Contribution from the Department of Chemistry, Auburn University, Auburn, Alabama 36849
Slow Electron Transfer between Main-Group Species: Oxidation of Nitrite by Chlorine Dioxide David M. Stanbury,*$+Roland Martinez, Ewen Tseng, and Charles E. Miller Received May 20,1988 A detailed study of the reaction of C102with NOT in aqueous media has been carried out. With excess NO2-the reaction gives C102-and NO< as products; Cloy reacts with NO2- to give CI- and NO3- in a slower phase. The rate law for the first phase is first order with respect to both [C102]and [NO2-],with k = 153 M-I s-’ at 25 OC, AH* = 56.5 f 1.1 kJ mol-’, and M*= -14 f 4 J mol-l K-I. Cloy inhibits the rate of the reaction, and its effect has been modeled by a mechanism involving reversible electron transfer to form NO2 and Cloy as primary products, followed by disproportionation of NO2. The electron-transferrate constant, although much less than diffusion controlled, is 3 orders of magnitude greater than that predicted by the cross relationship of Marcus’ theory, implying a strong-overlapmechanism.
Introduction Electron-transfer reactions involving simple main-group compounds have been a topic of longstanding interest in our laboratories. The general approach that has been taken is to investigate reactions between substitution-inert coordination complexes and the main-group substrates. The value of the approach is that the reactivity of the coordination complexes is well-known, which means that only one partner in the reaction, the main-group species, has largely unknown properties. By use of these methods it has been possible to determine the one-electron-reduction potentials for species such as Nj, NOz, and IS1-j It has also been shown that the rates of reactions involving couples such as S02/S02-,4 N3/N3-.’NOz/NO 3 with no added ClO, good pseudo-firstorder kinetics was observed. Semilog plots of the time-dependent absorbances, which were linear for at least 3 half-lives, were used to obtain the observed rate constants. In the studies with added Cloy the potential effects of the direct reaction of CIOz- with NO2- were minimized by mixing solutions of NO2- with solutions of C102plus C102-. When the reactions were conducted with [NOT] > 4 X 10” M, departure from the first-order dependence on [NO,] was seen. Under these conditions the order with respect to [NO,] was substantially greater than unity, and the absorbance changes were significantly less than expected, as though there were a rapid prior consumption of CIO,. Detailed investigation of these effects was hampered by the large and increasing absorbance due to NO2-; however, some blank tests were run that showed the effect to be independent of the observation wavelength, slit width, pH, buffer Ram, M. S.;Stanbury, D. M. J . Phys. Chem. 1986, 90, 3691-3696. Ram, M. S.:Stanbury. D. M. Inorg. Chem. 1985, 24, 2954-2962. Stanbury, D. M.; Wilmarth, W. K.; Khalaf, S.; Po, H. N.; Byrd, J. E. Inorg. Chem. 1980, 19, 2715-2722. Balahura, R. J.; Johnson, W. D. Inorg. Chem. 1987, 26, 3860-3863. Ram, M. S.;Stanbury, D. M. J. Am. Chem. Soc. 1984,106,8136-8142. Stanbury, D. M.; Lednicky, L. A. J . Am. Chem. SOC.1984, 106, 2847-2853. (a) McDowell, M. S.;Espenson, J. H.; Bakac, A. Inorg. Chem. 1984, 23,2232-2236. (b) Zahir, K.; Espenson, J. H.; Bakac, A. J. Am. Chem. SOC.1988, 1 IO, 5059-5063. Hoigne, J.; Bader, H. Vom Wasser 1982, 59, 253-267. Lednicky, L. A,; Stanbury, D. M. J . Am. Chem. SOC.1983, 105, 3098-3101,
0 1988 American Chemical Society
4278 Inorganic Chemistry, Vol. 27, No. 23, 1988 Table I. Kinetics Data from the Reaction of ClO, [NOT], M kob, s - ~ T, OC [NO,-], M 0.0040 0.55 25.0 0.072 0.080 0.83 25.0 0.0060 0.010 1.15 25.0 0.0080 0.020 1.45 25.0 0.010 0.020 2.9 25.0 0.020 0.020 4.9 25.0 0.032 0.020 0.040 6.6 25.0 0.020 8.7 25.0 0.052 0.020 10.8 25.0 0.060
with k,b, S-' 14.8 17.2 1.496 1.126 1.286 2.95b 4.396 6.45b 8.86b
Stanbury et al.
T,OC 25.0 25.0 25.0 13.2 15.8 25.3 29.9 35.1 39.9
100
a
50
-
e
0
U
o[C102]o = 4.3 X lo4 M, p = 0.2 M (NaC104), 1 mM P2074buffer, pH 6.0, 1.0-cm path length. [ClO,], = 2.6 X lo4 M, p = 0.5 M, pH 6.5. concentration, and added NO3-, C1-, or 0 2 . The second-order rate constant, activation parameters, and standard deviations were calculated by using the Los Alamos nonlinear leastsquares program. Numerical integrations were performed by using Hindmarsh and Byrne's subroutine EPISODE (Lawrence Livermore Laboratory, 1977). All calculations were performed on a Macintosh Plus computer.
Results Stoichiometry. In the prior study of the reaction between NOzand CIOz no results were reported concerning the products.8 In the present study the stoichiometry was examined by use of single-column anion chromatography with conductivty and amperometric detection. In one set of experiments a mixture of 8 X lo4 M N a N 0 2 and 5 X lo4 M CIOz was prepared and injected into the chromatograph after various time intervals. The first chromatogram showed formation of 2.3 X lo4 M CIOz- and a corresponding amount of NO3-. In subsequent chromatograms the ClO, peak diminished, the NO< peak increased, and a C1M peak appeared. In a second set of experiments at 5 X M CIOz the first chromatogram showed NaNO, and 5 X only a small amount of C10, but substantial amounts of C1- and NO3-. In subsequent chromatograms C102- was absent and the NO3- peak attained a stable intensity. Quantitative comparison of the peak heights with those of standard solutions indicated that the stoichiometry at long times is given as 2C102
+ 5N0, + H z O
-
5N03-
+ 2C1- + 2H+
(1)
It is apparent that CIOz- is formed at shorter times but that it reacts with NOz- on a longer time-scale to give the above stoichiometry. We can infer that at short times the stoichiometry is given by 2C1OZ
+ NOz- + HzO
-
2C1OZ-
+ NO3- + 2H+
(2)
The reaction of NO2- with CIOz- has recently been discussed by Beck and co-workers.IO Its rate law shows catalysis by H + and C1-. This behavior is consistent with our observations. Since our stoichiometry studies were performed with unbuffered solutions, and since H+ is a product in both reactions 1 and 2, we may assume that, under the buffered conditions of the kinetics studies described below, reaction 2 properly describes the process studied. Kinetics. The kinetics of the reaction between NO, and CIOz was studied at p = 0.2 and 0.5 M (NaClOJ, 25 OC. Solutions were buffered with 1 mM P207&,and the conditions generally held NO2- in large excess. The reactions were monitored by stopped-flow methods by observing the loss of absorbance due to C102. These were best observed at 360 nm, but at high concentrations of NO,- the background absorbance of NO2- was so high that results were determined a t 394 nm. Under most conditions excellent pseudo-first-order behavior was obtained; the data are listed in Table I. The reaction demonstrated an excellent first-order dependence on [NO2-] over the range from 4 to 40 mM. The rate of the reaction was independent of pH over the range from pH 7.5 to 4.9. It was not affected by ionic strength over the range 0.2-0.5 M, and additions of 1 mM ClO
