7307
The Reaction of Chloropentaaquoruthenium ( 111) with Chromium (11). Binuclear Intermediates, Reduction of Perchlorate, and the Effect of Vanadium (11) David Seewald, Norman Sutin, and Kay 0. Watkins Contribution from the Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973. Received June 13, 1969 Abstract: The reaction of chromium(I1) with chloropentaaquoruthenium(II1) in perchloric acid solution produces a chloride-bridged chromium(II1)-ruthenium(I1) intermediate. In the range (HC104) = 1.0 to 0.20 M the rate constant for the dissociation of the intermediate is given by k o b s d = a b/(H+) where a = 1.20 =k 0.06 sec-l and b = 0.36 f 0.03 M sec-l at 25.0'. The dissociation of the intermediate produces Cr3+and CrCIz+; the yield of CrC12+ is 88 f 5 % in 1.0 M perchloric acid and decreases with decreasing acidity. Vanadium(I1) catalyzes the disappearance of the intermediate. The rate constant for the reaction of vanadium(I1) with chloropentaaquoru0.06) x lo3 A4-l sec-' at 25.0" and is independent of acidity in the range (HCIOJ = 1.0 thenium(II1) is (1.88 to 0.10 M . The reaction of chromium(I1) with perchlorate ion is catalyzed by ruthenium(I1). The rate law for M-l sec-l at 25.0" and 1.0 M ionic this reaction is d(CrClz+)/dt = k[Ru(II)][CIO~-] where k = (3.3 A 0.3) X strength. The mechanisms of these reactions are discussed.
+
*
I
nner-sphere mechanisms have been established for many electron-transfer reactions. However, only in very few cases have the binuclear intermediates formed in these reactions been observed directly.*v3 The presence of these intermediates is usually inferred from rate comparisons, from the rate law, or from the identities of the products of the electron-transfer reaction. This paper is concerned with the binuclear intermediates formed in the reaction of chloropentaaquoruthenium(111) with chromium(I1). This system was chosen for study because of the relatively stable d6 and d 3 electron configurations of the products. The reaction of chloropentaaquoruthenium(II1) with vanadium(I1) and some of the properties of perchloric acid solutions containing ruthenium(I1) and chromium(I1) are also described. Experimental Section Materials. Solutions containing chloropentaaquoruthenium(II1) were prepared by refluxing a solution of 1.5 g of ruthenium trichloride in 50 ml of 0.1 M hydrochloric acid over 40 ml of mercury at 75" for 6 hr. The resulting dark brown solution was filtered and then stirred for 2 days with about 30 g of Dowex 5OW-X12 ion-exchange resin (2OC-400 mesh) in the H+form. The equilibrated resin was placed in a column of 1 in. diameter on top of about 2 in. of resin which had not been treated with the ruthenium(II1). A mixture of RuCla and RuC12+was eluted with 0.1 M perchloric acid and the RuCI2+ was eluted with 1.0 M perchloric a ~ i d . ~The , ~ eluate containing RuC12+was subjected to an additional purification step by diluting it tenfold with water, passing it through an ion-exchange column in the H+ form, and eluting the RuClz+with 1.0 Mperchloric acid. The RuC12+concentration was determined spectrophotometrically using e 670 at 317 nm. Total ruthenium was determined according to the procedure described by Ayres and Young.' The ruthenium concentration determined by the two methods generally agreed to within 5 x . Total chloride was determined as silver chloride by heating a solution of RuC12+with Ag+. The chloride determined in this pro(1) Research performed under the auspices of the U. s. Atomic Energy Commission, presented in part at the 155th National Meeting of the American Chemical Society, San Francisco, Calif., April 1968, Abstract MS. (2) H. Taube, Award Address, 153rd National Meeting of the American Chemical Society, Miami Beach, Fla., April 1967. (3) N. Sutin, Ann. Reu. Phys. Chem., 17, 119 (1966). (4) H. H . Cady, Ph.D. Thesis, University of California, 1957. (5) H. H. Cady and R. E. Connick, J . Amer. Chem. Soc., 80, 2646 (1958). (6) G. H. Ayres and F. Young, Anal. Chem., 22, 1281 (1950).
cedure was about 12z higher than the amount calculated from the ruthenium(II1) concentrations of the solutions. The additional chloride was formed in the oxidation of ruthenium(II1) by perchlorate during the analytical procedure. Solutions of RuCI2+gradually decompose upon standing at room temperature. The rate of decomposition increases with the RuCle+ and perchloric acid concentrations of the solutions. The RuCl*+ was therefore used as soon as possible after preparation. That fraction which was not used immediately was diluted tenfold with water, stored at 5 " , and purified by adsorption and elution from the ion-exchange column before use. Solutions containing chromium(I1) or vanadium(I1) were prepared by reducing chromium(II1) or vanadium(1V) with amalgamated zinc immediately before use. Chromium(I1)-vanadium(I1) mixtures were prepared by adding chromium(I1) to the vanadium(11) solutions. Other reagents were prepared and standardized as previously described.' Procedure. The oxidation-reduction reactions were studied by the use of the flow apparatus which is described elsewhere.* The disappearance of the intermediates was followed at 410 nm where the absorbance change was maximal. The following concentrato 4.0 X M; tion ranges were used: (Cr2+) = 8.0 X (RuC12+) = 1.33 X lo-' to 7.15 X lo-* M ; and (HClO4) = 0.1 to 1.0 M, and ionic strength 1.0 M. The rate constants were obtained from slopes of plots of log ( D t - Dm)us. time, where D t and D, are the optical densities at time f and at the end of the reaction, respectively. All of the kinetic measurements were made at 25.0". The yield of CrC12+in the RuC12+-Cr2+reaction was determined by flowing the reaction mixture through a 92-cm long polythene tube (i.d. = 3.9 mm) into a conical flask containing sufficient sodium persulfate to react with the excess chromium(I1). The quenched solution was diluted fivefold and placed on a Dowex 50W-X8 ion-exchange column. The column was washed with 0.2 M perchloric acid and the CrClz+was eluted with 1 M perchloric acid, Total chromium in the eluate was determined by the diphenylcarbazide methode and in some instances also as chromate (E4.83 X l o 3 at 372 nm) after oxidation with alkaline peroxide. The rate of formation of CrC12+in the ruthenium(I1)-catalyzed reduction of perchlorate by chromium(I1) was studied by flowing the RuCIZ+Cr2+reaction mixture into 25-1111 volumetric flasks in an argon atmosphere. At varying intervals excess sodium persulfate was added to the flasks to quench the reaction and the CrC12+ determined as described above. In later experiments the reaction mixture was flowed under argon into a IO-cm cell and the formation of chromium(II1) followed at 410 nm, using e 130 for the formation of CrCI2+(Ecrcla+ 7ecr3+). The perchlorate ion concentration of the
+
(7) A. Haim and N. Sutin, J . Amer. Chem. Soc., 88, 434 (1966). (8) G. Dulz and N. Sutin, Znorg. Chem., 2,917 (1963). (9) B. E. Saltzman, Anal. Chem., 24, 1016 (1952).
Seewald, Sutin, Watkins / Chloropentaaquoruthenium(IZI)with Cr(II)
7308 solutions was varied by maintaining the ionic strength with p toluenesulfonic acid (HF'TS).
Results and Discussion Chromium(I1)-Chloropentaaquoruthenium(II1) Reaction. The reaction between chromium(I1) and chloropentaaquoruthenium(II1) proceeds in two stages. The first stage proceeds too rapidly to be followed on the flow apparatus (k 2 2 X lo4 M-l sec-I in 1.0 M perchloric acid at 25.0"); the second stage, however, can readily be followed at 410 nm. The kinetic data are summarized in Table I. The first-order rate constants Table I. First-Order Rate Constants for the Disappearance of the Intermediate Produced in the Chromium(I1)-Chloropentaaquoruthenium(LII1) Reaction at 25.0'
(HClOA M
(Cr% X loa, M
(RuCl'+)o X 104, M
k, sec-l
1.oo 1.00 1.OO 1 .OO 1.00 1.00 1.00 0.91 0.85 0.75 0.75 0.70 0.60 0.60 0.60 0.50 0.50 0.49 0.44 0.40 0.40 0.34 0.34 0.30 0.30 0.29 0.26 0.26 0.25 0.24 0.20 0.19 0.15
40.0 25.9 24.0 24.0 15.6 8.0 8.0 24.0 15.6 15.6 24.0 24.0 24.0 15.6 24.0 24.0 15.6 40.0 24.0 40.0 15.6 40.0 40.0 15.6 24.0 24.0 15.6 40.0 24.0 24.0 24.0 15.6 24.0
5.30 5.30 3.60 1.60 5.30 2.85 1.33 2.45 2.65 5.30 3.60 2.45 2.45 2.65 3.60 2.80 5.30 2.50 7.15 2.50 2.60 2.70 2.50 5.30 2.80 7.15 5.30 2.50 2.80 7.15 3.65 5.30 3.65
1.57 1.44 1.60 1.52 1.49 1.53 1.47 1.58 1.60 1.59 1.81 1.75 1.73 1.78 1.93 2.00 1.98 1.97 2.13 2.04 2.04 1.97 2.13 2.36 2.46 2.53 2.46 2.26 2.67 2.66 2.86 2.81 3.30
AAbs" I(RuCIZ+)o 1.23 1.18 1.22 1.12 1.17 1.20 1.21 1.15 1.20 1.26 1.17 1.16 1.04 1.23 1.18 1.17 1.18 1.20 1.17 1.22 1.22 1.11 1.15 1.21 1.17 1.11 1.20 1.07 1.12 1.19 1.15 1.19 1.10
GAAbs is the total change in absorbance accompanying the decay of the intermediate at 410 nm, and I is the path length of the observation tube (1.9 mm).
calculated from the absorbance changes at 410 nm are independent of the chromium(I1) and ruthenium(II1) concentrations of the solutions provided (Crz+)o/(RuClz+)o 3 20, and increase with decreasing acidity. In the range (HC104) = 1.0 to 0.20 M l 0 the observed rate constant is given by
where a = 1.20 * 0.06 sec-' and b = 0.36 f 0.03 M sec-l at 25.0'. It is also apparent from Table I that (10) The first-order plots become curved (and wavelength dependent) at acidities below about 0.2 M . This curvature and wavelength dependence could be due to the formation of increasing amounts of a hydroxide-bridged species as the acidity is lowered.
Journal of the American Chemical Society
91:26
the value of AAbs/l(RuC12+)oat 410 nm, where AAbs is the total change in the absorbance accompanying the decay of the intermediate and 1 is the diameter of the observation tube, is independent of the initial concentration of RuC12+and also of the acidity provided the latter is greater than about 0.2 M. Both Cr3+ and CrC1*+ are produced in the reaction between chromium(I1) and chloropentaaquoruthenium(II1). The yield of CrC12+is 88 f 5 % in 1.0 M perchloric acid, approaches 100 % as 1/(HCI04) tends to zero, and decreases with decreasing acidity. In the range (HCI04) = 1.0 to 0.20 M the yield of CrC12+,i.e., the ratio of (CrC12+)/[Cr(III)] produced in the reaction, is given by
with c = 1.21 f 0.03 sec-l and d = 0.09 f 0.01 M sec-l at 25.0'. Although the kinetic measurements do not establish the composition or the structure of the intermediate, the stoichiometric data indicate that the intermediate formed at the higher acidities is [ R U ' ~ C ~ C ~ " ' ] ~ + . This result is not unexpected for neither ruthenium(I1) nor chromium(II1) undergoes substitution readily. The kinetic data are consistent with the following scheme. RuCP
+
[R,llCICrlll]r+
rapid
Cr2+ k
[ Ru~IC~C~"']~+
4 Ru?+
+
CrC12+
(3)
(4)
The increase in the rate of dissociation of the intermediate with decreasing acidity suggests a contribution from [Ru"CICr"']'+
+
H1O +==
+
[RU"CIC?~~OH]~' Ru2+
+
H'
Kh
(5)
Cr(OH)Cl+
(6a)
CrOH"
(6b)
[Ru"C~C~"'OH]~+ RuCl'
+
reactions 5 and 6. Reactions 3,4, 5 , and 6 can account for the principal features of the kinetic and stoichiometric results." In terms of the above scheme, a = c = kl, b = Khk2,d = K,k,, with kz = (k, kb).12 It will be seen that the experimental values of a and c are in good agreement. Comparison of the values of b and d gives k,/k, = 3. Evidently the ruthenium-chloride bond breaks at least 20 times more frequently than the chromium-chloride bond in [RuC1CrI4+. In [RuCICrOHI3+, on the other hand, the chromium-chloride bond breaks about three times more frequently than the ruthenium-chloride bond. Presumably the chromiumchloride bond is labilized by the hydroxide group which is attached to the chromium, an effect which has also been observed in other systems. These systems are compared in Table 11. The mercury(I1)-catalyzed aquations are included in this table, for the rates of these reactions are much less rapid than the rate of substitu-
+
(1 1) Since chromium(II1) is probably more highly hydrolyzed than ruthenium(II), it is likely that the hydroxide group is attached to the chromium(II1) in the hydrolyzed form of the intermediate. (12) This interpretation assumes that Kh/(H+)> 1 it follows that used in these studies.
Seewald, Sutin, Watkins
+ &-'[Cr(II)]) kobsd
= kl under the conditions
Chloropentaaquoruthenium(IIZ)with Cr(ZZ)
7310
The vanadium(I1)-catalyzed disappearance of [RIPC1Cr1I1I4+is a rather novel type of reaction. The added vanadium(I1) reacts with the RuC12+ existing in equilibrium with the [ R U ~ ~ C I C ~thereby I ~ ~ ] ~promoting + the dissociation of the binuclear species into reactants at the expense of its "normal" dissociation into products. In other words, although initially the vanadium(I1) fails to react with a significant amount of RuC12+because of the more rapid reaction of the latter with chromium(II), an appreciable fraction of the RuC12+is nevertheless consumed by the vanadium(I1) in the over-all reaction as a consequence of the relatively slow dissociation of [Ru"ClCrr1r]4+into products. The Individual Steps. Although the formation of [RuWIC~I'I]~+ from RuC12+ and chromium(I1) has been referred to as the first stage in the over-all reaction, this stage proceeds in two steps with k,k,/k-,k-, = ks + CrZ+e [ R u ~ I ~ C ~ C ~ ~ ~ ](11) ~+ 12-
RuC12+
8
The Chromium(I1)-Perchlorate Reaction. The decay of the intermediates produced in the reaction between chromium(I1) and chloropentaaquoruthenium(II1) is followed by a slower reaction producing CrC12+. The yield of CrC12+,i.e., the ratio (CrC12+)/Cr(III)produced in this reaction, is 12.5 f 1.0%. This stoichiometry conforms closely to that expected for the oxidation of chromium(I1) by perchlorate ions. In the presence of 8Cr(II)
+ C104- + 8H+ +CrCI*+ + 7Cr(III) + 4H20
excess chromium(I1) the initial rate of formation of CrC12+is given by d(CrC12+)/dt = k3[Ru(II)][C104-] (14) where k3 = (3.3 f 0.3) X M-' sec-l at 25.0" and ionic strength 1.0 M . The kinetic data are summarized in Table 111. The rate is independent of acidity in the range (HC104) = 0.15 to 1.0 M and is constant for the first 50% of the reaction after which it gradually dec r e a s e ~ . The ~ ~ kinetic data are consistent with a rate-
ke
[RLPCIC~II]~+
k-
e
[RuI1C1Cr1~IJ4+
(12) Table In. Second-Order Rate Constant for the Formation of
CrCP +in the Ruthenium(I1)-Catalyzed Reaction between l/K4. The first step involves substitution on chroChromium(I1) and Perchlorate Ions in 1.0 M Perchloric Acid mium(II), while the electron transfer from chromium(I1) at 25.0" to ruthenium(II1) occurs in the second step. The value (Cr2+h X (RuC12+)o X k X loa, of k, is probably about 3 X 10' M-' sec-', since this is M-l sec-' 103, M lo6,M the rate constant observed for substitution on chro50.Oa 35.1 3.3 mium(I1) in other oxidation-reduction reactions..14 10.5 1.22 3 .O Moreover the kinetic data give k,k,/k-, 3 2 X lo4 M-l 10.5 2.60 3.3 and k-, 3 sec-1; consequently k,/k-, 3 6.6 X 10.5 5.66 3.3 2.6 sec-'. This estimate of the lower limit of k-, is con10.5 10.4 3.5 10.5 31.2 3.8 sistent with the mechanism proposed for the vana27.0 5.65 3.4 dium(I1) effect for it shows that the dissociation of 8 . 50b 16.5 3.5 [ R U W ~ C ~into ~ ~reactants ~ ] ~ + can compete with its dis20.5O 25.4 3.6 sociation into products under suitable conditions. 20. 5d 25.4 3.8 The reduction potentials of the couples R U ( H ~ O ) ~ ~ + a Reaction quenched by the addition of persulfate. The CrClz+ e = RU(H*O)~*+ and RuC12+ e = RuClz are 0.2342 was determined after separation by ion exchange. * (HClOd = f 0.0088 and 0.01 f 0.03 V, respectively, in p-tol0.15 M ; (NaC10J = 0.86 M . C(HC104) = 0.50 M ; (HPTS) = 0.50 M. d (HClOa) = 0.20 M ; (HPTS) = 0.80 M. uenesulfonic acid at 25 ",2o If it is assumed that the reduction potential of the RuC12+IRuC1+couple is 0.05 V, and that the stability constant of RuC1+ is similar to determining step involving the reaction of perchlorate that of CrC12+(1.1 X lo-' M-' at 25°),21thentheequiwith ruthenium(I1) followed by the rapid reaction of librium constant of the over-all reaction chromium(I1) with the oxidized ruthenium and with RuC12++ Cr2+e RuZ++ CrClt+ chlorate ions. The second-order rate constant for the (13) ~______
+
+
+ ClOa- +Ru(1V) + c103+ 6Cr2+-+ CrC12+ + 5CrB+ Ru(1V) + 2Crz+ Ru(I1) + 2Cr3+
Ru(I1)
is calculated to be 5 X 10'. This value together with K4 M gives 7 X lo3 M for the equilibrium = 1.3 X constant of the reaction kl
e k-
+
C103-
---f
Ru2+-C104- reaction at 25.0" (3.3 X M-l sec-l) may be compared with 3.0 X M-l sec-' for the R U ( N H ~ ) ~ ~ + - Creaction ~ O ~ - ( p 0.62 M), and 1.6 X 10-2 M-' sec-l for the R U ( N H ~ ) ~ ~ + - Creaction ~ O ~ - ( p 0.2 M).24 It has been proposed that the rate-determining step in the latter reactions involves the transfer of an oxygen atom from perchlorate to r u t h e n i ~ m ( I I ) ,and ~~ it is very likely that a similar mechanism obtains also in the aquo system.
[ R U ~ I C ~ C ~ ~ Ru2+ ~~]~+ CrC12+ 1
Since kl = 1.20 sec-l, the value of k-l is calculated to be 2 X lod4M-' sec-l. Finally, comparison of k-l with k, indicates that ruthenium(I1) is about 10" times more substitution inert than chromium(II), or, put somewhat differently, the average lifetime of a water molecule in the inner coordination sphere of ruthenium(I1) is estimated to be several minutes at 25". This estimate is consistent with some qualitative observations on the (22) E. E. Mercer and R. R. Buckley, Znorg. Chem., 4, 1692 (1965). (23) This decrease in rate could be due to the formation of rutheniumrate of substitution reactions involving the R U ( H ~ O+) ~ ~(111) species which are not rapidly reduced by chromium(I1). For ion.22 example, reactions such as Ru(1V) + Ru(I1) -,[Ru(III)]zcould become (20) R. R. Buckley and E. E. Mercer, J . Phys. Chem., 70,3103 (1966). (21) R. J. Baltisberger and E. L. King, J , Amer. Chem, Soc., 86, 795 (1964).
Journal of the American Chemical Society
1 91:26
increasingly important as the chromium(I1) concentration decreased, and could account for the rate decrease if the reaction between [Ru(III)]2 and chromium(I1) is relatively slow. (24) J. F, Endicott and H. Taube, Inorg. Chem., 4, 437 (1965).
December 17, 1969
