3389
REACTIVITY OF Co(II1) COMPLEXES IN AQUEOUS SOLUTIONS
Reactivity of Some Cobalt(111) Complexes toward Photochemically Produced Hydrogen Atoms and Solvated Electrons in Aqueous Solution1
by John F. Endicott and Morton Z. Hoffman Department of Chemistry, Boston University, Boston, Massachusetts 02216
(ReceCed January 14, 1966)
Aqueous I - solutions of ICo(NH3)e 1(C104)3, [Co(“3) SOH21(C104)3, [Co(NH3)4(OH2)& (cio4)3,and [Co(NHa)&l](ClO& were photolyzed a t 2537 A and the rate of Co(I1) and 1 3 - production was determined as a function of [Co(III)], [H+], temperature, and light intensity. The I- was the most strongly absorbing species present in each experiment. hv --t (I-e-)aq was scavenged by Co(II1) and H+. The eaq- formed in the reaction IThe quantum yield of Co2+production approaches a different limiting value a t high and low [H+]. The ammine and aquo complexes each show the same reactivity toward eaq-, consistent with the suggestion that these reactions are diffusion controlled. All four complexes exhibit a very low reactivity toward the reducing species formed in the irradiation of very acidic solutions.
+
Introduction There has been a great deal of recent interest in the very rapid reactions of metal ions and complex ions with electrons2-‘ and with H atom^^-^ in aqueous solution; these reactions with cobalt(II1)-ammine complexes are the most rapid ever reported. It has been suggesteds-l0 on theoretical grounds that the reactions of ea,- with cobalt(II1)-ammines are diffusion controlled, depending only on the collision frequency of the positive and negative ions. If the Marcus-Sutin argument is correct and the’ reaction of es,- is diffusion controlled, then substitution of C O ( N H ~ ) , O H ~or ~+ C O ( N H ~ ) ~ ( O H as ~ ) the ~ ~ +oxidizing agent should not alter the rate of the reaction despite the fact that the aquo complexes are better oxidizing agents than the hexaammine.” This prediction of the similarity in rates is substantially in agreement with the work of Baxendale, et u1.,I2published while the present investigation was in progress. The reduction of these complexes by Ha, provides an interesting comparison to the ea,- case. Although the former are still far more rapid than the reduction of Co(II1) by most known reducing agents,13 they are several orders of magnitude slower than the reduction by eaq- 5-7 and show differences in rate for the various Co(II1) complexes. These Co(III)-Haq reactions are sufficiently slow that they probably are not diffusion
controlled. For these slower reactions, the nature of the ligands coordinated to the Co(II1) center would be expected to play a much more important role in determining the reactivity of the complex. It has been reported516that C O ( N H ~ ) Breacts ~ + more rapidly wit,h Ha, than does C O ( N H ~ ) ~ O H which ~ ~ +is opposite (1) Presented before the Division of Physical Chemistry, 151st National Meeting of the American Chemical Society, Pittsburgh, Pa., March 1966. This research was supported in part by NSF through Grant GP-3467and in part by the Graduate School of Boston University. (2) M. Anbar and E. J. Hart, J . Phys. Chem., 69, 973 (1965). (3) M. Anbar, “Solvated Electron,” Advances in Chemistry Series, No. 50,American Chemical Society, Washington, D. C., 1965,p 55. (4) E. J. Hart, Science, 146, 19 (1964). (5) G. Navon and G. Stein, J . Phys. Chem., 69, 1390 (1965). (6) M. Anbar and D. Meyerstein, Nature, 206, 818 (1965). (7) J. Halpern and J. Rabani, J . Am. Chem. Soc., 88, 699 (1966). (8) R.A. Marcus, paper presented at IAEA Symposium on Exchange Reactions, Brookhaven National Laboratory, June 1965. (9) N. Sutin, paper presented a t IAEA Symposium on Exchange Reactions, Brookhaven National Laboratory, June 1965. (10) R. A. Marcus, “Solvated Electron,” Advances in Chemistry Series, No. 50, American Chemical Society, Washington, D. C., 1965, p 138. (11) R. G. Yalman, Inorg. Chem., 1, 16 (1962). (12) J. H. Baxendale, E. M. Fielden, and J. P. Keene, Proe. Roy. SOC. (London), A286, 320 (1965). (13) For example, J. F. Endicott and H. Taube, J . Am. Chem. Soe., 86, 1686 (1964).
Volume 70, Number I 1
November 1966
3390
to the usual order of reactivity for these oxidizing agents. l 3 An important difference between the esq- and Ha, reductions of Co(II1) complexes may result from the kinds of activated complex formed by these two reducing agents, the former undoubtedly being of the “outer-sphere” type while the latter may be of the bridged type. There is another mechanism by which H atoms often react with organic substrates, hydrogen abstraction, l 4 which is generally not possible in reactions of inorganic substances. However, a hydrogen abstraction mechanism has been suggested15to account for the OH,, reduction of C0(“3)6~+ and Co(NH3)BOHz3+. This study was undertaken to investigate Co(II1)ea,- reactions under carefully controlled conditions of ionic strength and temperature so that they might be compared directly with other chemical reductions of Co(II1) complexes performed under the same conditions. We chose to use the 2537-A irradiation of I&,-as the means of producing eaq- because this technique provides an efficient source of eaq-.16 Scavengers react with eaq- thus produced according to diffusion controlled nonhomogeneous kinetics. l6 However, the scavenger concentration is sufficiently great in the present investigation to obtain the limiting yield of eaq-. This can be determined quantitatively and is not subject to the restrictions of diffusion kinetics. Hydrogen atoms are also produced in the Iaq- photolysis16 due to scavenging of eaq- by H + and so a means was available to attempt to observe the Co(111)-Haq reaction under these same controlled conditions. As we had found in some preliminary work,17 the competition of Co(II1) and H + for eaq- is actually reflected in the rate of production of Co(I1). Cobalt(II1)-ammine complexes are exceptional scavengers for very reactive reducing species because the oxidation potentials of the complexes are extraordinarily sensitive to the nature of the coordinated ligands.11s18 Furthermore, the Co(I1) products are in very rapid equilibrium with the solution so that the reduction of Co(II1) complexes in acidic solution have always been found to be irreversible and free of the complications due to back reactions.
Experimental Section All experiments were carried out at 2537 A. The lamp, reaction cell, actinometry, preparations of compounds and solutions, and the Co(I1) analysis have all been described previously.“ Solutions were thermostated by immersing the lamp and cell in a water bath regulated at 25 or 60”. The solution to be irradiated was purged of dissolved Oz by the sweeping of a stream The Journal of Physical Chemistry
JOHN F. ENDICOTT AND MORTON Z. HOFFMAN
of Nz which had been passed through a solution of Cr2+. The gas from the photolysis cell was vented to the atmosphere through a syringe needle in a serum cap. For each run at high light intensity, at least four samples were taken at 15-sec intervals. At the low light intensities, the interval was approximately 1 min. The initial rate of production of Co2+,Ri, was determined from a plot of [Co2+]vs. time of irradiation. Over the very small extents of reaction observed in the great majority of the determinations, the [Co(III)] did not change sufficiently in order for large (>15%) deviations from linearity to appear in the zero-order plots. Although the effective order in [Co(III)] is somewhat changed for reactions in dilute acid which were more than 30% complete in 1 min due to the competition reactions, the calculated initial rates are believed to be reliable to within 10-15%. For the reactions involving very dilute solutions of Co(NH3)&l2+ and for some studies in highly acidic solutions of C O ( N H ~ ) [Co2+] ~ ~ + ~was determined by the extraction of CO(SCN)~ with methyl isobutyl ketone. The KHbSCN solution was extracted prior to its use.5 Solutions of known [Co2+]were used for the spectrophotometric calibration. Iodine formed was determined by measuring the absorption of Is- at 3530 A. Aliquots of solutions to be analyzed for 1 3 - were mixed with I- and HC1 so that [I-] ‘v 0.5 M and [H+] ‘v 0.1 M . The molar absorbancy of 13- a t 3530 A was taken as 261400.19 I n some cases, the total concentration of oxidant in the irradiated solution was determined by the iodidemolybdate method.20 Uranyl oxalate actinometry was performed at each of the light intensities and temperatures reported in this study. For the higher temperature actinometry, the published temperature dependence of the quantum yield of the actinometer solutionz1 and our own determination of the temperature variation of the absorp-
(14) S. W. Benson, “The Foundations of Chemical Kinetics,” McGraw-Hill Book Co., Inc., New York, N. Y., 1960. (15) D. Katakis and A. 0. Alien, J . Phys. Chem., 68, 1357 (1964). (16) G. Stein, “Solvated Electron,” Advances in Chemistry Series, No.50,American Chemical Society, Washington, D. C., 1965,p 230. (17) J. F. Endicott and M. Z. Hoffman, J . Am. Chem. Sac., 87, 3348 (1965). (18) W. Latimer, “Oxidation Potentials,” 2nd ed, Prentice-Hall, Inc., Englewood Cliffs, N. J., 1952. (19) A. D.Awtrey and R. E. Connick, J . Am. Chem. Sac., 73, 1842 (1951). (20) C. J. Hochanadel, J . Phys. Chem., 56, 587 (1952). (21) W.A. Noyes, Jr., and P. A. Leighton, “The Photochemistry of Gases,” Reinhold Publishing Corp., New York, N. Y., 1941.
3391
REACTIVITY OF CO(II1) COMPLEXES IN AQUEOUS SOLUTIONS
ance of the uranyl oxalate solution at 2537 A were used. To minimize problems connected with the absorption of the radiation by Co(II1)-I- ion all solutions used in this study were dilute in both Co(II1) M) and I- (8 X lop3M ) . Under these (~(OH~)Z~+ solutions. In the case of the C O ( N H ~ ) ~ Csolutions, I~+ the high absorptivity of the complex at 2537 A limited the absorption of the radiation by the I- to 70-85% even with very dilute (-1 X M) Co(II1) solutions. Except where otherwise noted, the ionic strength of all the solutions was maintained at 0.11 M by the addition of NaC1O4.
Results and Discussion
and because the observations at low acidity can be interpreted in a very straightforward manner, the investigations at high and low acidities will be discussed separately. A . Low Acidity. The primary absorption process under our experimental conditions has been considered to lead to the production of a “geminate pair” consisting of an I atom and cas- l6
I-
+ hv -+
*I- + (1-e-)
(4Ia) In the presence of electron scavengers, such as H+ and cobalt(II1)-ammine complexes, 23 the reaction of the scavenger with the geminate pair Co(II1) H+
+ (1-e-)
--j
I
+ Co2+
+ (1-e-) + (I-H)
(1) (2)
will compete with the “geminate recombination” reaction.16 (1-e-) -+ I-
We find that the 2537-A irradiation of acidic I- solutions containing Co(II1) always leads to the form& tion of Co2+ and 13-. The quantum yield of Co2+ formation, (PcO2+, depends upon the acidity of the solution and approaches a limiting value at high [H+] as shown in Figure 1. We have attempted to fit our data, obtained in the range pH 1-3, to several different mechanisms, but find that the same mechanism cannot quantitatively account for our observations at the two extremes of pH. Because the predominant reducing species may change from eaq- to Ha, as the acidity increases,’*
(3) At low [€€+], our data are quantitatively represented by reactions 1 and 2 scavenging for all the electrons produced in the primary absorption step. Under such conditions of total scavenging, the steady-state rate law is
-
d [Co2+] -- 4 I a h [Co(III) I dt - kl[Co(III)] kz[H+]
+
(A)
As this simple competition would predict, Ia/Ri is linearly correlated to [H+]/ [Co(III)] at constant [H+] (Figure 2) and the y intercept is greater than zero; the value of 4 obtained from this intercept (0.26 f 0.05) agrees well with the value of 0.29 obtained by Jortner, Ottolenghi, and Steinz4for the limiting quantum yield of electrons produced by the 2537-A irradiation of aqueous I-. The slopes of the lines in Figure 2 decrease systematically with increasing [H+], implying that at high [H+] reduction of Co(II1) occurs through some reactions other than (1). If the values of these slopes are extrapolated to [H+] = 0, the ratio lcz/kl ‘v 0.8 is obtained. The s~~~~~ value of kz/kl = 0.25 from radiolysis s t ~ d i e was ~~
I
0
0.02
I
0.04 0.08 [H+l, M .
I
0.08
I
0.1
Figure 1. Variation of *coz+ with f H + ] for Co(NHa)s(C10& a t 25”. Point X is the limiting quantum yield for the generation of solvated electrons from aqueous I - solutions 3.4 x 10-8; according to Stein;Z4 I , in einstein L-1 min-1: f, 0,0.90 X lo-’; 0, 0.23 X lo-’; [Co(NHa)s+’] = 1.0 X 10-3111; fi = 0.11 M ; [I-] = 0.008 M.
(22) M.G.Evans and G. H. Nancollas, Trans. Faraday SOC.,49, 363 (1949). (23) In the general discussion, the symbol Co(II1) will be used to represent any of the oxidants employed in this study: CO(NH~)~S+, or Co(NHa)sClz+. Co(NHs)sOHta+, Co(NHa~4(OHz)z~+, (24) J. Jortner, M. Ottolenghi, and G. Stein, J . Phys. C h a . , 66, 2029 (1962). (25) J. H. Baxendale, E. M. Fielden, and J. P. Keene, Proc. C h a . Soc., 242 (1963). (26) L. M. Dorfman and I. A. Taub, J . Am. C h a . SOC.,85, 2370 (1963); S. Gordon, E. J. Hart, M. S. Matheson, J. Rabani, and J. K. Thomas, Discusewfls Faraday SOC.,36, 193 (1963).
Volume ‘YO, Number 11 November 1066
JOHN F. ENDICOTT AND MORTON Z. HOFFMAN
3392
Table I: Product Yields from Co(NH&3+ Solutions a t [H+] = 1.1 X 1 0 - * M "
40
0.16 0.31 0.92 1.84
0.039 0.067 0.13 0.17
0.021 0.042 0.047 0.065
a 2 ' = 25'; p = 0.11 M ; [I-] = 0.008 M ; I. = 3.4 X einstein 1.-l rnin-l. Interpolated from data in Figure 2.
0 0
5
10
15
lH+l/[Co(III) I. Figure 2. I./& us. [H+]/[Co(III)] for C O ( N H & ~ +at 25". [H+] inM: 1.1 X 10-3; 0,5.4 X 0, 1.0 X IO-*. p = 0.1.1 M ; [I-] = 0.008 M . The values of I. are the same as indicated in Figure 1 but are not distinguished in this figure because of the first-order dependence of Ri on I,.
+,
obtained a t much lower ionic strengths than employed in this study so that the difference in the cationic charge types is expected to be more significant. In a few experiments at ionic strengths in the range (2-15) X M , when [Co(III)] M , we found 4 0.19 and kz/kl >/ 0.32, which is consistent with this argument. One additional factor which complicates the interpretation of the low ionic strength studies in this system is that 4 should decrease markedlyI6 as the solutions become dilute in electron scavengers. As seen in Table I, @cO2+'v 2 @ ~ - .This good stoichiometric agreement, when taken with the fact that our values of Cpco2+are in good agreement with Stein's2* limiting quantum yield of eaq- production from I-, Ieads us to conclude that reduction of the Co(II1) complexes by a species such as Iz- does not occur to any appreciable extent in these systems. In Stein's work,24at a greater [I-] than that which we have used, an appreciable yield of I2 was observed at pH 1, presumably through I- scavenging.27 (1.H)
+ I- + + H
-
Table I1 : +c0z
+
for the Ammine and Aquo Complexes" [CO(IW I
M
[H+l X 103. M
*eo2
CO(NH~)B~+
0.41 0.41 0.37 0.49
1.1 5.4 11 110
0.074 0.031 0.017 0.014
CO(NH,)~OH~~+
0.41 0.44 0.44 0.54
1.0 5.4 21 110
0.059 0.044 0.032 0.019
C0("3)r(OH2)2+
0.52 0.52 0.52 0.51
3.3 11 22 110
0.076 0.047 0.037 0,030
X 108,
Complex
' T = 25'; p = 0.11 M; [I-] = 0.008 M ; I, einstein 1. -1 min-1.
=
+
3.4 X
12-
At Our [I-'' this reaction appears to be unimportant* The general quantitative agreement with Stein's The Journal of Physical Chemistry
investigations of I- photolysis also rules out significant oxidation by iodine of very short-lived cobalt(I1)ammine species. The data in Table I1 are representative of our studies of the ammine and aquo complexes in acidic solution. We find that in solutions of low acidity ([H+] 'v to M ) , C O ( N H ~ )and ~ ~ +C O ( N H ~ ) ~ O are H~~+ about equally reactive toward the predominant reducing species, (I e-). The data seem to indicate that of the three complexes, C O ( N H ~ ) ~ ( O H ~is) , ~con+ sistently most reactive toward any reducing species, but a t pH 3, and allowing for the effect of direct photolysis, this complex is not more than 30% more efficient than Co(NH3)e3+in scavenging for (1.e-). B. High Acidity. When [H+] >> [Co(III)], H + scavenges for all the eaq- so that the principal
>
