R. K. MURMANN, H. TACBE AND F. *I. POSEI'
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L'ol. 71)
changing the distribution of Co(II1) between the readily adopt S N 1 mechanism^,^^^?^ because a reactant RHzO+++ and the activated complex means of stabilizing a penta-coordinated interR(H20)2+++. The outer-sphere affinity of these mediate, by making use of unshared electron pairs two forms for sulfate ion is expected t o be ap- on the remaining group X, is available. proximately the same, and the present state of Acknowledgment.-The research described was understanding of these effects would accommodate supported by the Office of Naval Research under a slight preference in either direction. contract KG-Ori-02026. Funds for the purchase of The conclusions we have reached concerning the mass spectrometer were supplied by the Atomic mechanism for ions of the type C O ( N H ~ ) ~ H ~ O +Energy ++ Commission under contract At(l1-1)-92. and Co(NH3),Xf+ provide no basis for conclusions (19) D. D. Brown and C. I(.Ingold, J . Chem. SOL.,2680 (19331. in other systems, or for the reactions under widely ( 2 0 ) R . G. Pearson, C R. Boston and F. Basolo, THISJorJRK.41., 76, difTerent conditions. Thus the reactants of the 3089 (1953). tetrammine series Co(NH3)&+ may much more CHICAGO 37, ILLISOIS
[CONTRIBUTION FROM GEORGEHERBERT JOXES LABORATORY, UXIVERSITY
OF
CHICAGO]
Mechanisms of Electron Transfer in Aquo Cations-The Reaction of RH,Offf with Cr++ BY R. K. MURMANN, H. TAUBE AND F. A. POSEY RECEIVEDJULY 5, 1956
A kinetic study of the reaction of C r + + with ( N H ~ ) s C O H Z O +to + +form Cr(H20)6+++and C o y + has been made, tracer experiments on the transfer of oxygen from ( X H ~ ) ~ C O H Z O + to+ C + r + + have been done, and isotope fractionation effects on the oxygen and nitrogen in (NH3)5CoHzO+++during reaction have been measured. The kinetic data are consistent with the rate law -d(Cr++)/dt = kl(Cr+~)((NH3)aCoH2OC++) i- k2(Cr++)((NH3)jCoOH++). At 25" and p = l., kl = 19 1. The reaction 2 X lo7 mole-' min.-'. E1 = 8.7 kcal. mole-' and al = 108.7 1. mole-' min.-l. mole-' min.-', and k2 corresponding to the second term in the rate law proceeds by substantially complete transfer of oxygen from the oxidizing agent to the reducing agent. For the reaction corresponding t o the first term, it can be stated t h a t a t least partial oxygen transfer is involved. The fractionation factor d In O16/d In 0l8for the oxygen in the bridging ligand OH- is 1.034; and a substantial fractionation for nitrogen is also indicated. T h e efficiency of C1- acting as a bridging ligand and as a simple ligand in the activated complex of composition Co( NH3)5+++.CI-.Cr + + is compared.
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T h e preparations R H ~ 0 ( C 1 0 4 )RH~0*(C104)3~ ~, and C r + + aq 2 were made following procedures described elsewhere. The rate of the reaction of C r + + and RHzO+++ was followed by discharging known amounts of the reaction mixture into F e + + + in excess, and determining the amount of Fe formed, by amperometric titration. The tracer experiments were done by allowing C r + + t o r e x t with R H 2 0 + + +in excess, in perchloric acid solution,
precipitating the excess Co(III), the Zn(I1) (formed in the reduction of Cr(II1)) and Co(I1) ions, then precipitating Cr(&O)e+++ as the phosphate,G heating this precipitate t o obtain HzO, and finally determining the isotopic composition of the H20 by the COz method.' At the concentrations of RHzO+++ and of HC104 necessary to provide for the formation of Cr(HzO)e+++ in reasonable quantity, the system contained solid RH20(C10a)3during most of the time the reaction proceeded. For the fractionation work R ( H 2 0 ) + + + was used in excess. After substantially complete reaction with C r + + , the residual R ( H 2 0 ) + + + was precipitated as the bromide. This salt was washed, dried and heated to yield water. Comparison of the final oxygen-isotope composition of the RHzO+++, with that of the salt initially, taking into account the fraction left unreacted, yielded values for the isotope fractionation .* I n connection with the experiments on the kinetics, which were completed toward the end of the research, there was evidence t h a t precise stoichiometry in the reaction did not always obtain. Apparently some process for the destruction of C r + + which does not involve Co(II1) complex can interfere and the contribution of this additional reaction is not easily controlled. These effects have been more strikingly revealed in recent experiments in which C r + + acts as catalyst rather than by being consumed in the reaction under study, and here the net destruction of C r + + , presumably by C104-, but catalyzed by adventitious substances could be readily demonstrated. The effect described was almost completely absent in the experiments on the kinetics which have been reported, although it appears to have been present in earlier, preliminary work using a different batch of C r + + . It should be mentioned that different batches of C r + + show the side reactions to a differing extent, but other factors may also influence the extent t o which it contributes.
(1) H. Taube, H . Myers and K. L. Rich, THISJOURNAL, 76, 4118 (1953). ( 2 ) H. T a u b e and H. Myers, ibid., 76, 2103 (1954). (3) I
