J. Phys. Chem. 1986, 90, 3823-3829 is large, the more realistic Morse curVe21b representing the stretching of the C-X bond in the reactant differs substantially from the approximating parabola not only under standard conditions but also in the range of driving forces where the heterogeneous and homogeneous electron transfers were experimentally investigated. A Morse curve description of the reactant could be used for improving the activation model but the actual difficulty then resides in the description of the R X- potential energy surface. The improvement of the model thus hinges upon the development of an accurate quantum mechanical description of X- system as a function of their distance. the R
+
+
Concluding Remarks The main conclusions that emerge from the preceding discussion can be summarized as follows: In the case of aromatic halides, the kinetic data gathered so far indicate that the reduction cleavage of the carbon-halogen bond involves the intermediacy of the anion radical of the starting molecule.35 There is a rough correlation between the cleavage rate constant of the anion radical and the ArX/ArX- standard potential. Since the latter is an approximate representation of the cleavage free energy, this correlation (which exhibits a transfer coefficient close to 0.5) can be viewed as a Bronsted-Marcus activation-driving force free energy relationship. The transition state corresponds to the unpaired electron passing from the T * orbital to the g* orbital of the C-X bond. In contrast, aliphatic halides undergo a strictly speaking dissociative electron transfer as suggested by the low value of the electrochemical transfer coefficient and confirmed by the analysis of the whole set of heterogeneous and homogeneous kinetic data. The latter are reasonably well fitted by the activation-driving force relationship based on Marcus quadratic theory, even though this is a grossly approximate representation owing to the large magnitude of the activation energies. Upon passing from I to Br to C1 the reduction becomes more difficult from two additive factors: the standard potential shifts negatively essentially because of the increase of the carbon-halogen bond energy; the activation free energy increases essentially because the force required to stretch the bond concomitantly becomes larger and larger.
3823
Acknowledgment. We thank Drs. D. Zann and I. Gallardo for their active contribution to the study of the aromatic and butyl halides, respectively. Collaboration with Prof. A. Merz (University of Regensburg, BDR) in the investigation of 9-chloro-9-mesitylfluorene was essential. Registry No. n-BuI, 542-69-8; n-BuI'-, 101980-38-5;n-BuBr, 10965-9; n-BuBr'-, 102045-61-4;n-BuCI, 109-69-3;n-BuCl'-, 77347-44-5; sec-BuI, 513-48-4; sec-BuI'-, 102045-62-5; sec-BuBr, 78-76-2; secBuBr'-, 102045-63-6; sec-BuCI, 78-86-4; sec-BuCl'-, 102045-64-7; tert-BuI, 558-17-8; tert-BuI'-, 53487-00-6; tert-BuBr, 507-19-7; tertBuBr'-, 57422-69-2;tert-BuCI, 507-20-0; tert-BuCI'-, 102045-65-8;p NOZC~H~CI, 100-00-5;p-N02C6H,CI'-, 34473-09-1;p-CIC&C(O)Ph, 134-85-0;p-C1C6H4C(O)Ph'-, 81439-06-7;p-ClC,H,CN, 623-03-0;p CIC6H4CN'-, 68271-91-0;p-CIC6H4C(O)CH,, 99-91-2;p-CIC6H4C(O)CH3'-, 68225-77-4;m-C1C6H4C(0)CH,,99-02-5;m-CIC&C(O)CH3'-, 68225-76-3; o-NO~C~H~CI, 88-73-3;o-N02C6H4CIS-,34470-274; p-BrC6H4No2,586-78-7;p-BrC6H4N02'-,34470-26-3;p-BrC6H4C(O)Ph, 90-90-4;p-BrC6H4C(0)Ph'-, 57365-05-6;p-BrC6H4C(0)CH,, 99-90-1; p-BrC6H4C(0)CH3'-,34473-43-3; m-BrC6H4C(0)CH3,214263-4; m-BrC6H4C(0)CH3'-,77510-39-5;m-BrC6H4C(0)Ph,1016-77-9; m-BrC6H4C(0)Ph'-, 101980-39-6; 9-chloroanthracene, 716-53-0; 9chloroanthracene radical anion, 74430-88-9;4-chloroquinoline, 6 1 1-35-8; 4-chloroquinoline radical anion, 101980-40-9;2-chloroquinoline, 61 262-4; 2-chlorquinoline radical anion, 7 1803-43-5;2-chloronaphthalene, 9 1-58-7; 2-chloronaphthalene radical anion, 5 1703-41-4; 1-chloronaphthalene, 90-13-1; 1-chloronaphthaleneradical anion, 51703-40-3; 2-chloroanthracene, 17135-78-3; 2-chloroanthracene radical anion, 91503-57-0; (E)-4-[2-@-chlorophenyl)ethenyl]pyridine, 46459-15-8; (E)-4-[2-@-chlorophenyl)ethenyl]pyridine radical anion, 102045-66-9; 1-chloroanthracene, 4985-70-0; 1-chloroanthracene radical anion, 7149-70-4; I-bromo-291 503-58-1; l-bromo-2-methyl-4-nitrobenzene, methyl-4-nitrobenzeneradical anion, 34470-34-3;1-bromo-2-isopropyl5-nitrobenzene, 101980-41-0;l-bromo-2-isopropyl-5-nitrobenzene radical 94832-09-4; anion, I O 1980-42-1; 1-bromo-2-isopropyl-4-nitrobenzene, I-bromo-2-isopropyl-4-nitrobenzeneradical anion, 101980-43-2; 9bromoanthracene, 1564-64-3;9-bromoanthraceneradical anion, 549 1 151-2; 1-bromonaphthalene, 90-11-9; 1-bromonaphthaleneradical anion, 51703-42-5; 2-bromo-l,3-dimethyl-5-nitrobenzene,53906-84-6; 2bromo-l,3-dimethyl-5-nitrobenzeneradical anion, 101980-44-3; 3bromo-9H-fluoren-9-one, 2041- 19-2; 3-bromo-9H-fluoren-9-one radical 36804-63-4; l-bromoanion, 101980-45-4;l-bromo-9H-fluoren-9-one, 9H-fluoren-9-oneradical anion, 101980-46-5.
Rate-Structure Dependencies for Intramolecular Electron Transfer via Organic Anchoring Groups at Metal Surfaces Michael J. Weaver* and Tomi T.-T. Li Department of Chemistry, Purdue Unifiersity, West Lafayette, Indiana 47907 (Received: January 21, 1986: In Final Form: April I , 1986)
The electroreduction kinetics of pentaamminecobalt(II1) complexes that are surface-attached to mercury or gold electrodes via extended thioorganic linkages featuring nitrogen Co(II1) coordination are examined and compared to similar systems involving oxygen coordination sites. These comparisons utilize unimolecular rate constants, k,, (SI), and preexpotential factors for the elementary elebtron-transfer step, together with thermodynamic adsorption data and rate constants for the homogeneous outer-sphere reduction of the complexes by Ru(NH,),~+. These data enable the observed rate variations with the bridging ligand structure arising from changes in the activation free energy to be separated from those due to variations in the electronic transmission coefficient, K,,. This analysis indicates that the latter provides the predominant component of the ca. 105-fold observed variations in ket. All nonconjugated, and even some conjugated, organic bridges yield significantly or substantially nonadiabatic pathways (Le., K,, 10 V S-I, voltammetric sweep rates so that the faradaic current arises from initially adsorbed, rather than diffusing, Co(II1) reactant.3a) Values of the precursor stability constants, K p (cm), for each reactant at mercury electrodes are also listed in Table I. As before,3athese were determined from K p = rp/cb,where rp(mol cm-2) is the equilibrium surface reactant concentration determined from the total faradaic charge contained under the rapid linear sweep voltammograms employed to measure ket, and C, is the corresponding bulk reactant concentration (usually 0.02-0.05 mM). These K p values refer to the initial potential (200 mV) at which the adsorbed layer was formed prior to the rapid cathodic voltammetric weep.^^,^ Roughly comparable values of K p can
+
(4) Dockel, E. R.; Everhart, E. T.; Gould, E. S. J. Am. Chem. Soc. 1974, 88, 5661. (5) (a) Balahura, R. J.; Wright, G. B.; Jordan, R. S. J. Am. Chem. Soc. 1973,95, 1137. (b) Balahura, R.J. J. Am. Chem. SOC.1976, 98, 1498. (c) Wang, Y.-I.; Gould, E. S. J. Am. Chem. SOC.1969, 91, 4998.
The Journal of Physical Chemistry, Vol, 90, No. 16, 1986 3825
Electron Transfer via Organic Anchoring Groups
TABLE I: Summary of Kinetic Data for the Reduction of Com(NH&L at Mercury and Gold Surfaces at -200 mV vs. SCE and by Ru(NH&*+ at 24 OC kobsdrC kRu{ AH*,g ligand L surface 10-2k.,," s-l a.! cm s-l anhd Kn: cm M-I s-I kJ mol-! A.,," s-] 10-2ket' SKI 9
I I1
I11 IV V
VI
VI1 VI11
6 O C N -
s
