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NMA, which also shows marked autoprotolysis. The autoprotolysis constant of NMF measured by a spectrophotometric method harmonizes well with the corresponding values of other amidic solvents and allows the study of protolytic equilibria with reference to SH2+-SH or SH-S- standard pairs.
References and Notes (1) L. R. Dawson in "Chemistry in Nonaqueous Ionizing Solvents", Vol. IV, G. Jander, H. Spandau, and C. C. Addison, Eds., AkadembVerlag, East Berlin, 1963, p 259. (2) C. M. French and K. H. Glover, Trans. FaradaySoc., 51, 1418 (1955). (3) R. P. Held and C. M. Crlss, J. Phys. Chem., 8Q,2611 (1965). (4) E. J. Klng in "The International Encyclopedia of Physical Chemlstry and Chemical Physics", E. A. Guggenheim, J. E. Mayer, and F. C. Tompkins, Eds., Pergamon Press, 1965. (5) R. P. Bell, "The Proton in Chemisby", Comell University Press, Ithaca, NY, 1973, Chapter 4. (6) L. Weeda and G. Somsen, Red. TraV. Chim. Pays-Bas, 85, 159 (1966). (7) A. Finch, P. J. Gardner, and C. J. Steadman, J . Phys. Chem., 71, 2946 (1967).
(8) C. M. Criss, R. P. Held, and E. Luksha, J. Phys. Chem., 72, 2970 11968). (9) 6.L. be Ligny, H. J. M. Denessen, and M. Alfenar, Red. Trav. Chim. Pays-Bas, 90, 1265 (1971). (10) D. S.Gill, S. J. P. Singla, R. Ch. Paul, and S.P. Neruia, J. Chem. Soc., Dalton Trans., 522 (1972). (11) R. M. Meighan and R. H. Cole, 2, Phys. Chem., 68, 503 (1964). (12) M. Kitano and K. Kuchitsu, Bull. Chem. SOC.Jpn., 47, 631 (1974). (13) F. W. Fowler, A. R. Katrfky, and R. I.D. Rutherford, J. Chem. Soc. B, 460 (1971). (14) S.J. Bass, W. I. Mathan, R. M. Meighan, and R. H. Cole, J. Phys. Chem., 68, 509 (1964). (15) T . Miyazawa, T. Shimanouchl, and S.Mizushima, J. Chem. Phys., 24, 408 (1956). (16) A. J. Parker, J. Chem. Soc., 1328 (1961). (17) A. Berger, A. Loewensteln, and S.Meiboom, J . Am. Chem. Soc., 81, 62 (1959). (18) 0. Fraenkel and C. Franconi, J. Am. Chem. Soc., 82,4478 (1969). (19) R. J. Gillespie and T. Birchall, Can. J . Chem., 41, 148 (1963). (20) W. E. Stewart and T. H. Siddall, Chem. Rev., 70, 517 (1970). (21) 6. G. Cox, J. Chem. SOC. B, 1780 (1970). (22) Reference 4, Chapter 11. (23) C. D. Ritchie in "Solute-Solvent Interactions", J. E. Coetzee and C. D. Ritchie, Eds., Marceli Dekker, New York, 1969.
Electron Exchange between Ferrocene and Ferrocenium Ion. Effects of Solvent and of Ring Substitution on the Rateli2 Edward Shih Yang, Man-Sheung Chan, and Arthur C. Wahl" Department of Chemistry, Washington University, St. Louis, Missouri 63 130 (Received: June 2, 1980)
The rates of electron exchange between bis(cyclopentadienyl)iron(II) and -(III) (ferrocene and ferrocenium ion) and between oxidized and reduced forms of several derivatives of ferrocene have been measured in a number of different solvents by the NMR line broadening method over a temperature range of 0-30 O C . It was found that the rates did not vary with the dielectric properties of the solvents as predicted by the Marcus theoretical model for electron exchange between neutral and singly charged spherical reactants with similar structures, reactions for which solvent reorganization is the principal deterent to exchange. Also, the product of the collision number (2)and the transmission coefficient ( K ) was found to be an order of magnitude smaller than the generally assumed value of KZ= lo1' M-l s-l. Addition of NaPFBor NaC104to acetonitrile solutions of ferrocene and ferrocenium ion reduced the exchange rate by about a factor of 2 at high (0.1-0.5 M) salt concentrations. The presence of substituents on the cyclopentadienyl rings affected the rate of electron exchange only moderately; the largest effect of about a 10-fold increase in rate was observed for the decamethyl derivative. The presence of the methylenetrimethylamine group on one ring, resulting in 1+ and 2+ charged reactants, caused a reduction in rate by a factor of -5, much less than the factor of -60 estimated for Coulombic repulsion between uniformly charged spheres 7 A in diameter, an indication that the charged quaternary amine groups are widely separated (> 10 A) in the transition state. Electrical conductivity measurements of cobaltocenium hexafluorophosphate in a variety of solvents indicated that it and very probably the similar ferrocenium salt are essentially completely dissociated at the low concentrations used for measurement of electron-exchange rates in most solvents investigated.
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Introduction The rates of electron exchange between ferrocene (Fe(CP)~)and ferrocenium ion (Fe(Cp),+) and between their deviatives in various solvents are of interest because the main deterent to electron transfer is probably the necessity for solvent re~rganization.~ There is no Coulombic repulsion between the reactants, so the work required to bring them together is minimal, and their structures are similar, the distance between rings being 3.3 f 0.1 A for both Fe(Cp)z6and Fe(Cp)z13,eso the energy required for internal rearrangement should be small and essentially the same for electron exchange in different solvents. Therefore, investigation of the rate of electron exchange in different solvents should increase our understanding of the requirements for solvent reorganization. 0022-3654/80/2084-3094$01 .OO/O
and examination of Consideration of bond space-filling models indicates that the iron is buried between the cyclopentadienyl rings, which nearly touch and thus shield the iron from solvent molecules. The main solvent interactions, therefore, are probably with the two rings, which together form the surface of a molecule or ion having a cylindrical shape, the length and diameter being approximately equal. Thus, the exchange systems investigated resemble, to a reasonable approximation, the simple model proposed by Marcus' of spheres in a continuous, unsaturated dielectric medium, and comparison of experimental results with theoretical predictions should be informative. A number of solvents with a considerable range of dielectric constants were used, and, to investigate the pos0 1980 American Chemical Soclety
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Electron Exchange between Fe(Cp), and Fe(Cp),+
sibility of association between Fe(Cp),+ and the counterion, PF,, we made conductivity measurements for the similar but much mlore stable Co(Cp),PF6 compound in the various solvents. No evidence of ion association was found except for thie chlorinated hydrocarbon solvents, CH2Cl2 and CHzCICHzC1,and also to a small extent for CHsOH. Addition of KPF6 or KClo4to an acetonitrile solution of reactants alffeded the rate only moderately;the rate was lowered about a factor of 2 at the highest KPF6 or KClO4 concentrations of 0.1-0.5 M. If the effect is attributed entirely to ion-pair formation, electron exchange involving an ion pair is -2 times slower than it is when the free ion is involved. 'Therefore, small changes in electrolyte concentration at the low ionic concentrations (10-3-10-2 M) present in most reaction solutions should have affected the exchange rates negligibly. Early inves)tigationsof the Fe(Cp),-Fe(Cp),+ exchange reaction were reviewed, and our results obtained with CH3CN and CH30H as solvents were reported previously.' The general rnethods of measurement, data analysis, and compound purification were also reported in the previous articles8
Experimental Section The NMR data were analyzed by use of the line-width equation, as described WDP = f p w p f (1- fP)WD + f P ( 1 - fP)4r(6V)'/(kc) (1) The symbols WP, WD,and WDp represent the full widths at half-maximum of the resonance absorption peaks of the paramagnetic, diamagnetic, and mixtures of the two species (W = (TT,)-~); 6v, tz, and c represent the contact shift, the second-order rate constant for exchange,1° and the total reactant concentration (c = cp + cD), respectively, and f p = cp/c. For all systems investigated exchange was in the fast limit (kc :>> 27r(6v)), and, except for the n-butyl derivative discuseed below, the protons €or which absorption was measured were equivalent and uncoupled to other protons,11so colnditions were appropriate for use of eq For the n-butyl derivative, the a and /3 protons were coupled (Jaa= 7 f 1Hz), but exchange between the two components is in the fast limit, and correction for coupling is only 1Hz, or
