to the National Science Foundation for financial support. The purchase of the Varian n.m.r. instrument was made possible by a grant from the National Science Foundation. The cooperation of Drs. C. U. Pittman, Jr., George Olah, and Herman G. Richey, Jr., was most helpful. N. C. Deno, Jane S. Liu, John 0. Turner David N. Lincoln, Richard E. Fruit, Jr. Department of Chemistry, Whitmore Laboratory Pennsylvania State University, University Park, Pennsylvania I6802 Received April 14, 1965
Transient Intermediates in Inner-Sphere Electron-Transfer Reactions Involving Polyatomic Bridging Ligands Sir: One of the important questions which arises in connection with the mechanisms of inner-sphere electron-transfer reactions involving polyatomic bridging ligands, e . g . , in the reductions of Co(NH3),X2+ (where X = CN, NOz, NCS, etc.) by reductants such as Cr2+(aq)' or C O ( C N ) ~ ~2 , -3 to give Cr(HzO),X2+
A
B Figure 1. Stopped-flow oscillograms showing transient interby- pentaamminecobalt(II1) mediates in the oxidation of C O ( C N ) ~ ~ complexes at 25". (A) oxidant, C O ( N H & N O ~ ~(1.0 + X lW3 M ) ; 2.0 X l W 4 M Co(CN),3-; 0.0125 M CN-; wave length, 380 mp; time scale, 2 sec./major division. (B) Oxidant Co(NH&CN2+ (2.0 X M ) ; 2.0 x 10-8 M Co(CN)s3-; 3.0 X MCN-; wave length, 320 mp; time scale, 1 sec./major division.
and Co(CN),X3-, respectively, relates to the position of attack of the reductant on the bridging ligand, and to the configuration of the initial oxidation product which in some cases is thought to differ from the final product. It has been ~ u g g e s t e d ,for ~ example, that the reduction (1) H. Taube, J . Am. Chem. SOC.,77, 4481 (1955). (2) J. P. Candlin, J. Halpern, and S. Nakamura, [bid., 85, 2517 (1963). (3) J. Halpern and S . Nakarnura, "Proceedings of the 8th Conference on Coordination Chemistry, Vienna," Springer Verlag, Berlin (W.), 1964, p. 271. (4) R . L. Carlin and J. 0. Edwards, J . Inorg. Nucl. Chem., 6 , 217 (1958).
3002
Journal of the American Chemical S o c i e t y
87:13
of (NH3)jCoNCS2+by Cr2+(aq) involves attack at the S atom, followed by electron transfer via the inter[(NH3)~Co-NCS-Cr(H20)j]4+, to yield mediate the S-bonded complex (H20)6Cr-SCN2+as the initial oxidation product ; but the latter, if formed, apparently rearranges to the stable N-bonded isothiocyanato product (H20)jCr-NCS2t too rapidly to be detected. The studies reported here are pertinent to this theme and appear to be the first examples in which direct evidence for the position of attack in such reactions has been obtained through the detection of the transient initial oxidation products. The studies in question relate to the oxidation of C O ( C N ) ~ ~by - several pentaamminecobalt(II1) complexes, C O " ~ ( N H ~ ) ~ XAs . previously shown,213 the general rate law for these reactions has the form - d[CO"'( N H 3) sX]/d t = ki[CO''I( N H 3) ,XI[ C O( CN) j "1 k, '[Co"'(NH 3)5X][Co( CN) ,3-][CN-], the first term corresponding to the inner-sphere oxidation of Co(CN)b3- through the bridged intermediate [(NH3)5Co-X-Co(CN)b], to CO"'(CN)~X, and the second term to the outer-sphere oxidation of CO(CN),~- to Co(CN)e3-. For ( N H 3 ) 5 C ~ - N 0 ~(containing 2+ an N-bonded nitro ligand), as oxidant, the values of ki and k,', at 25" and p = 0.2, were found to be 3.4 X lo4 M-I set.-' and 3 X IO4 M-z sec.-l, respectively. At 0.0125 M CN- about 99% of the oxidation of Co(CN)j3- proceeds by the inner-sphere (ki) path to yield Co(CN),NOn3- (NOn also N-bonded to Co) as the final product. Stopped flow measurements2 (Figure 1A) clearly reveal that under these conditions the disappearance of C O ( N H ~ ) , N O ~(tl12 ~ + = 0.1 sec.) does not coincide with the formation of the final product but rather of a transient intermediate which decays much more slowly (tl12 = 7 sec.) to CO(CN)~NOz3-. The transient exhibits strong absorption in the wave length region (-380 mp, Figure 1A) where both CO(NH~)~NO and ~ ~Co(CN),NOn3+ are nearly transparent, but which is characteristic of pentacyanocobaltate(II1) complexes containing 0-bonded ligands (e.g., C O ( C N ) ~ O H ~A,~ - 380 mp, E,,, 240). These observations are consistent with attack of CO(CN),~on an oxygen atom of the bridging NOz ligand, resulting in electron transfer through the intermediate [(NH3),Co-N02-Co(CN),]- to produce 0-bonded CO(CN)~ONO~initially, and subsequent isomerization of the latter to Co(CN),NOn3-. The first-order rate of isomerization was unaffected by increasing the CO(CN)~~ concentration from 1 X lo-, to 2 X M , suggesting that the isomerization under these conditions occurs intramolecularly rather than via ligand transfer between CO(CN)~ONO 3and Co(CN), '-. Similar evidence for a transient intermediate was obtained with Co(NH3),CN2+ (C-bonded)6 as oxidant. The values of ki and k,' for this oxidant at 25" and p = 0.2 were found to be 2.9 X lo2 M-l set.-' and 4.5 X IO3 M - 2 set.-', respectively, both paths in this case giving rise to CO(CN),~- as the final product. At 0.03 M CN-, with the inner-sphere path accounting for some 70% of the reaction, the disappearance of Co(NH3),CN2+ (ill* = 0.8 sec.) coincided with the
+
( 5 ) R. B. Penland, T. J. Lane, and J. V. Quagliano, J . A m . Chem. SOC.,78, 887 (1956). (6) H. Siebert, 2. anorg. allgem. Chem., 327, 63 (1964).
1 J u l y 5 , 1965
ki appearance of a transient (Figure 1B) which absorbed trans-PtHCI(PEt& + pyridine e k-i more strongly than CO(CN)c3- at 320 mp (consistent trans-PtH(pyridineXPEt~~+ + C1- (1) with a pentacyanocobaltate(lI1) complex containing an N-bonded ligand) and which underwent first-order is some lo5 times as great as that of the corresponding decay to C O ( C N ) ~ ~with - a half-life of 1.6 sec. These pyridine substitution of t r a n ~ - P t C l ~ ( P E t ~In ) ~ .view of observations are consistent with the sequence: (NH3)5the magnitude of this rrans effect and of the important Co-CN2+ C O ( C N ) ~(4 ~ - [(NH~),CO-CN-CO(CN)~~)questions of interpretation associated with it, it ap--t Co(CN)5-NC3- .-t Co(CN)E3-. peared of interest to measure the kinetic isotope effect Thus it appears that, in each of the above cases, on the rate of reaction 1 arising from the deuterium attachment to the bridging ligand occurs at an atom substitution of the hydride ligand. We report here the which exposes an unshared electron pair to the attacking results of these measurements which reveal an unusually reductant even when this position of attack does not large secondary kinetic isotope effect. lead to the thermodynamically stable product. trans-PtHCl(PEt3)2 was prepared by the method of It is also significant in this context that similar experiChatt and Shaw,2 using hydrazine as the reducing ments failed to reveal any evidence for analogous agent. The corresponding deuterido compound, Iranstransient intermediates in the oxidation of Co(CN)b3PtDCl(PEt3)z having an isotopic purity of 93 %, was by two other oxidants, C O ( N H ~ ) ~ - O N O(~O+- b ~ n d e d ) ~ made by exchanging the hydrido compound with and C O ( N H ~ ) ~ - N C S ~(N-bonded + isothiocyanate),' CzHsOD using DCI as catalyst. Successive batches of where the thermodynamically stable products Co(CN)bthe hydrido and deuterido complexes, obtained by such NOz" and CO(CN)~-SCN~-(S-bonded thiocyanate)8 repeated exchanges, yielded reproducible results. The can be formed directly by electron transfer through the kinetic measurements were made in anhydrous ethanol intermediates [(NH3)5C~-0zN-C~(CN)6]and [(NH3)5solution, the reaction being followed conductometrically CO-NCS-CO(CN)~]-, generated by attack of Co(CN)b3according to the method of Basolo, et u1.l The on a ligand atom having an exposed electron pair. reaction was found to be reversible and the kinetic Acknowledgment. Support of this work by the data fitted the rate law3 National Science Foundation (Grant G P 654) is grate- d[PtHCI(PEt&]/dr = kl[PtHC1(PEt&](pyridine) fully acknowledged. k-l[PtH(pyridineXPEt&+l[C1-] (2)
+
(7) M. Linhard, R. Siebert and M. Wiegel, Z . anorg. allgem. Chem.,
278, 287 (1955). (8) J. L. Burmeister, Inorg. Chem., 3, 919 (1964).
Jack Halpern, Shuzo Nakamura Department of Chemistry, The University of Chicago Chicago, Illinois Received M a y 12, 1965
A Secondary Kinetic Isotope Effect in a Substitution Reaction of a Square-Planar Complex
Sir: It has been reported' that the hydride ligand in trans-PtHC1(PEt3)z exerts a strong labilizing influence
Values of kl were computed from this rate law using a value of 0.030 for the equilibrium constant ( = kl/k-l) of the reaction, determined by the method of Basolo, et ul.,' and in good agreement with the value found by them. Over the range of our measurements (Table I), this value was, within experimental error, independent of the isotopic substitution and of the temperature. The results of our kinetic measurements are summarized in Table I and reveal a remarkably large secondary kinetic isotope effect, corresponding to a decrease in kl by a factor of approximately 1.4 when the hydride ligand is replaced by deuteride. The activation parameters of the reaction, derived from
Table I. Summary of Kinetic Data Temp.,
"C. 0 0 0 0 0 0 0
- 10 - 20 - 10
-20
Initial concentrations, M [PtHCI(PEt&$ [Pyridine]
x
104
2.0 4.0 8.0 4.0 4.0 8.0 8.0 4.0 4.0 2.0 2.0
x
101
1.2 1.2 1.2 3.6 6.0 3.6 6.0 6.0 6.0 1.2 1.2
-kl,"
M-I sec.-l-
H
ki(Wlki(D)
Dc
1.20 1.16 1.08 1.05 1.08 1.06 1.06 0.503 0.209 0.490 0.219
1.73 1.60 1.55 1.56 1.60 1.50 1.52 0.702 0.295 0.718 0.317
Mean kinetic isotope effect
1.44 1.37 1.44 1.48 1.48 1.42 1.43 1.40 1.41 1.47 1.45
1.44~f 0.05
Or [PtDCl(PEt&]. * Each of these rate constants is the average of approximately four independent determinations, the reproducibility of which was generally better than 4~4%. c Corrected for the isotopic purity (93%) of the PtDCI(PEt&.
(trans effect) on the chloride ligand, this being reflected the observation that the rate Of reaction l
'
(1) F. Bas&, J. Chatt, H.B. Gray, R. G . Pearson, and B. L.Shaw, J . Chem. Soc., 2207 (1961).
(2) J . Chatt and B. L. Shaw, ibid., 5075 (1962). (3) No contribution to the forward rate of reaction 1 could be de-
__
tected from a Dvridine-indeuendent Dath. The contribution from such a path reported earlier' appears to be an artifact arising from failure to take account of the back reaction in treating the kinetic data.
Communications to the Editor
3003
