Henry Taube Nobel Lecture
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136 Chemistry 1983 absorption confers on Prussian Blue its characteristic blue color. The interva- lence absorption is at longer wave length for species I than for Prussian Blue because the two iron sites in the latter are not substitutionally equivalent. This leads to a ground state energy difference which is then added to that associated with the Franck-Condon barrier when the process is induced by a quantum of light. One of my main interests in the field of mixed valence molecules has been to explore and to try to understand the energetics of the systems. I will illustrate by a single example the kind of conclusion which we have reached in pursuing these interests and where we have relied on theory introduced into the field by Hush (85), by Mulliken (90), and for the correlation of extent of electron delocalization with electronic structure, Mayoh and Day (91), and choose for illustration the localized mixed valence molecule (92) The stability conferred on the ground state of the molecule by charge delocali- zation is only of the order of 50 cal, far below the upper limit of 5 × 10 2
the electrochemical results, which measure the total stabilization of the mixed valent compared to the isovalent state. When the nuclear coordinates about each center are adjusted so that the Franck-Condon condition is met, the energy separating the bonding and anti-bonding states which arise from elec- tron delocalization is calculated as 2.2 kcal (93). This is taken to be sufficient to ensure adiabatic transfer (94) in agreement with the conclusion reached in the course of studying intramolecular electron transfer in Co(III)-Ru(II) systems with 4,4’-bipyridine and related molecules as bridging ligands (76). If electron transfer is assumed to be adiabatic, the specific rate for intramolecular electron transfer is calculated as 3 × 10 8
-1 , in reasonable agreement with an estimate ( l . 6 × 1 0
8 s - 1 ) reached from measurements of intermolecular electron transfer rates for pyrinedinepentaammineruthenium species (95). Mixed valence molecules have been prepared (96) which are delocalized even though the bridging group is so large that direct metal-to-metal orbital overlap cannot be responsible for the delocalization. These have remarkably interesting properties in their own right, and are the subject of current studies (97). CONCLUDING REMARKS In this paper I have focussed rather narrowly on electron transfer reactions between metal complexes. The separation of this subclass from other possible ones which can be assembled from the reactant categories: metal complexes, organic molecules (98) molecules derived from other non-metallic elements, any of the above in excited states (99, 100), electrodes, is not totally arbitrary as it might be were it dictated solely by limitations of space. Admittedly, all the possible electron transfer processes are governed by the same principles, at least when these are stated in a general enough way. But as these principles H. Taube 137
manifest themselves in the different subclasses, the descriptive chemistry can be quite different, and these differences are the fabric of chemistry. The subclass which has been treated has perhaps been the most thoroughly studied, yet, as the article by Sutin shows (36), our understanding at a basic level is far from complete. Even for the much studied Fe 3+ /Fe 2+ self exchange reaction, which served to introduce this subject, the important question of whether the reaction is adiabatic or not has not been settled to everyone’s satisfaction. Still a great deal of progress has been made. The descriptive matter has increased enormously since 1940, and our understanding of it, both in scope and depth, has more than kept pace with observations. A great deal of progress has also been made in many of the other subclasses - for example in the study of electrode processes, and in “atom transfer” reactions (as a specific case the use of transition metal species to carry the oxidizing capacity of O 2 to a substrate such as an organic molecule). Both are of the greatest importance in industrial applications, and the latter also in reaching an understanding of the chemistry of living cells. Because the subclasses are interrelated, progress in one leads to progress in another. ACKNOWLEDGEMENT Many of the co-workers who have contributed to progress in the subject of this article are cited in the references, and this is an implicit acknowledgment of their contributions. Because this account is incomplete, others who have contri- buted directly to this work have not been cited, nor still others who have had interests that are not reflected in the account I have given. I am grateful to them all for the help they have given, and for what I have learned in the course of working with them. The nature of the contributions made by my co-workers is not evident either from the acknowledgment I have made, nor from my expression of gratitude. I need to add that I have always relied on the independence of my co-workers and to a large extent my contribution to the effort has been that of maintaining continuity. I also wish to acknowledge financial support of my research by the agencies of the U. S. Government, beginning with the Office of Naval Research in about 1950. Later, I derived partial support from the U. S. Atomic Energy Commis- sion, and still later from the National Science Foundation and the National Institutes of Health (General Medical Sciences). The Petroleum Research Fund of the American Chemical Society has also been a source of research support.
138 Chemistry 1983 REFERENCES with the specific rates of the component “self exchange” reactions Fe Allowance is made in the equation for the effect of driving force on the rate. (6) Dodson, R. W., J. Am Chem. Soc. (1950) 72, 3315. (7) Silverman, J.; Dodson, R. W., J. Phys, Chem. (1952) 56, 846. (8) Lewis, W. B.; Coryell, C. D.; Irvine, S. W., Jr. J. Chem. Soc. (1949) S386. (9) The descriptor “inner sphere ” is used for a reaction in which oxidant and reductant metal centers are linked through primary bonds to a bridging group. In the early work it was assumed that the bridging group would play a special role in the electron transfer reaction. In more recent work on intramolecular transfer (vida infra) examples have been found ofsystems in which the bridging group seems to serve only to hold the two partners together. “Inner sphere” or “outer sphere”? (10) Holstein, T., J. Phys, Chem. (1956) 56, 832. (11) Libby, W. F., J. Phys. Chem. (1956) 56, 863. (12) As a historical note, I wish to add that James Franck was very much interested in electron transfer in chemical reactions and fully appreciated the importance of keeping the Franck- Condon restriction in mind in trying to understand the observations (13) Brown, H. C., discussion of the paper of ref. 11, p 868. (14) Libby, W. F., discussion following the paper of ref. 11, p 866. See also Brown, H. C., discussion following paper of ref. 7, p 852. (15) Dodson, R. W.; Davidson, N., discussion following the paper of ref. 11, p 866. (16) Taube, H.; Myers, H.; Rich, R. L., J. Am. Chem. Soc. (1953) 7.5, 4118. (17) Taube, H.; Myers, H., J. Am. Chem. Soc. (1954) 26, 2103. (18) Taube, H., Chem. Rev. (1952) 50, 69. (19) The subject of ionic hydration is brought up to date in an article by J. P. Hunt and H. L. Friedman, p 359, ref 1. (20) Bjerrum, J.; Paulsen, K., Nature (1952) 169, 463. (21) Eigen, M., Faraday Soc. Discussions, (1954) No. 17, p 1. (22) Bjerrum, J., Z. Physik. Chem. (1907) 59, 336, 581; Z. Anorg. Allgem. Chem. (1921) 119, 179. (23) Hunt, J. P.; Taube, H., J. Chem, Phys. (1950) 18, 757; ibid (1951) 19, 602. (24) Meredith, C. W., U. S. Atomic Energy Rept. UCRL-11704 (1965) Berkeley. Work done under supervision of R. E. Connick. (25) Candlin, J. P.; Halpern, J., Inorg. Chem. (1965) 4, 766 (26) Taube, H.; King, E. L., J. Am. Chem. Soc. (1954) 76, 4053. (27) Ball, D. L.; King E. L., J. Am. Chem. Soc. (1958) 80, 1091. (28) Snellgrove, R.; King, E. L., J. Am. Chem. Soc. (1962) 84; 8609 (29) Taube, H., J. Am. Soc. (1955) 77, 4481. (30) Ogard, A. E.; Taube, H. J., Am. Chem. Soc. (1958) 80, 1084. (31) Gjertsen, L.; Wahl, A. C., J. Am. Chem. Soc. (1959) 81, 1572; Sheppard, J. C.; Wahl, A. C., J, Am. Chem. Soc. (1957) 79, 1020. (32) Sutin, N.; Gordon, B. M., J. Am. Chem. Soc. (1961) 83, 70. (33) Marcus, R. A., Ann. Rev. Phys. Chem. (1964), 15, 155; also earlier papers. (34) Hush, N. S., Trans. Faraday Soc. (1961) 57, 557. (35) Consider the process: I II H. Taube 139
State I is the precursor complex; in state II the energy is independent of whether the electron is on atom A or B, i. e., the Franck-Condon condition has been met. In adiabatic transfer, electron delocalozation is great enough so that whenever state II is reached, electron transfer can take place, and the rate of the chemical reaction is determined solely by the rate at which state II is reached. In non adiabatic transfer, the system passes through state II a number of times before electron transfer occurs, and both the Franck-Condon barrier, and the rate of electron transfer in state II are rate determining. (36) Sutin, N., p 441 of ref. 1. (37) Orgel, L. E., Rept X e Conseil Inst. Intern, Chem. Solvay (1956) 289. (38) George, P.; Griffith, J., Enzymes (1959) 1, 347. (39) Halpern, J.; Orgel, L. E., Discussions Faraday Soc. (1960) 29, 32. (40) Zwickel, A. M.; Taube. H., J. Am. Chem. Soc. (1959) 81, 2915. (41) Halpern, J.; Nakamura, S., J. Am. Chem. Soc. (1965) 87, 3002. (42) Dulz, G.; Sutin, N., J. Am. Chem. Soc. (1964) 86, 829. (43) Connochioli T. J.; Nancolles, G. H.; Sutin, N., J. Am. Chem. Soc. (1969) 86, 1453. (44) Connochioli T. J.; Nancolles, G. H.; Sutin, N., J. Am. Chem. Soc. (1964) 86, 459. (45) Sykes, A. G.; Thorneley, R. N. V., J. Chem. Soc. (1970) A232. (46) Espenson, J. H., J. Am. Chem. Soc. (1967) 89, 1276. (47) Price, H. J.; Taube, H., Inorg. Chem. (1968) 7, 1 (48) Haim, A.; Sutin, N., J. Am. Chem. Soc. (1965) 87, 4210. (49) Fronaeus, S.; Larsson, R., Acta Chem. Scand. (1962) 16, 1447. (50) Espenson, J. H.; Birk. J. P., J. Am. Chem. Soc. (1965) 87, 3280; ibid, (1968) 90, 1153. (51) Kruse, W.; Taube, H., J. Am. Chem. Soc. (1960) 82, 526 (52) Toppen, D. L.; Linck, R. G., Inorg. Chem. (1971) 10, 2635. (53) Basolo, F.; Morris, M. L.; Pearson, R. G., Disc. Faraday Soc. (1960) 29, 80. (54) Anet, F. A. L.; Leblanc, E., J. Am. Chem. Soc. (1957) 79, 2649. (55) Espenson, J. H.; p. 189 of the volume of ref. 1. (56) Last in the series. Fraser, R. T. M. and Taube, H., J. Am. Chem. Soc. (1961) 83, 2239 (57) I owe my associates during my first years at Stanford an enormous debt of gratitude for helping to set the record straight. Special thanks are due to E. S. Gould, who first uncovered discrepancies, and to J. K. Hurst who repeated much of the dubious earlier work. (58) Nordmeyer, F. R.; Taube, H., J. Am. Chem. Soc. (1966) 88,4295; ibid (1968) 90, 1162. (59) The relation between reducibility of the ligands, and their effectiveness in mediating electron transfer was developed in an earlier paper. Gould, E. S.; Taube, H., J. Am. Chem. Soc. (1964) 86, 1318. (60) Sebera, D. K.; Taube, H., J. Am. Chem. Soc. (1961) 83, 1785. (61) See, for example: Gould E. S., J. Am. Chem. Soc. (1972) 94, 4360. (62) Endicott, J. F.; Taube, H., J. Am. Chem. Soc. (1962) 84, 4989; ibid (1964) 86, 1686; Inorg. Chem. (1965) 4, 437. (63) Gleu, K.; Breuel, K., Z. Anorg. Allg. Chem. (1938) 237, 335. (64) Gaunder, R.; Taube, H., Inorg. Chem. (1970) 9, 2627. (65) Anderson, A.; Bonner, N. A., J. Am. Chem. Soc. (1954) 76, 3826. (66) Indirect evidence, which is quite convincing, suggests the outer sphere self exchange rate for to be ~ 5 x 1 0 - 1 0
M - 1
s - 1
. M e l v i n , W . S . ; H a i m . A . , I n o r g . C h e m . (1977) 16, 2016. (67) Meyer, T. J.; Taube, H., Inorg. Chem. (1968) 7, 2369. (68) Stynes, H. D.; Ibers, J. A., inorg. Chem. (1971) 10, 2304. (69) Calculations of the barrier associated with inner sphere electron reorganization leave room for a non-adiabaticity factor of a few orders of magnitude. See Endicott, J. F.; Krishan, K.; Ramasami, T; Rotzinger, F. P., p 141 of ref. 1. (70) Isied, S. S.; Taube, H., Inorg. Chem. (1976) 15, 3070. (71) Meyer, T. J.; Taube, H., Inorg. Chem. (1968) 7, 2361. (72) Harrison, D. E.; Taube, H., J. Am. Chem. Soc. (1967) 89, 5706. (73) Robson, R.; Taube, H., J. Am. Chem. Soc. (1967) 89,6487; French, J.; Taube, H., J. Am. Chem. Soc. (1969) 91, 6951, earliest example: Saffir, P., J. Am. Chem. Soc. (1960) 82, 13. 140 Chemistry 1983 (74) Hoffman, M. Z.; Simic, M., J. Am. Chem. Soc. (1972) 94, 1957. (75) (a) Isied, S. S., J. Am. Chem. Soc. (1973) 95, 8198. (b) In an earlier effort, Kirk Roberts tried the simple procedure of mixing the Co(III) complex with (NH 3 ) 5 R u O H
2 2+ 2+
Substitution is too slow relative to intramolecular transfer for . the method to work in these systems. (76) Fischer, H.; Tom, G. M.; Taube, H., J. Am. Chem. Soc. (1976) 98, 5512. (77) Schäffer, L. - work in progress. (78) Dixon, N. E.; Lawrance, G. A.; Lay, P. A.; Sargeson, A. M., Inorg. Chem. (1983) 22, 846. (79) Gaswick, D. G.; Haim. A., J. Am. Chem. Soc. (1974) 96, 7845. (80) Haim, A. see p 273 of ref. 1. (81) Winkler, J. R.; Nocera, D. G.; Yocom, K. M.; Bordignon, E.; Gray, H. B., J. Am. Chem. Soc. (1982) 104, 5798. (82) Isied, S. S.; Worosila, G.; Atherton, S. J., J. Am. Chem. Soc. (1982) 104, 7659. (83) The level of current activity in the field can be gauged by the recent review of the subject of mixed valence molecules based on couples by C. Creutz: see ref. 1, p 1. (84) Allen, G. C.; Hush, N. S., Prog. Inorg. Chem. (1967) 8, 357. (85) Hush, N. S., Prog. Inorg. Chem. (1967) 8, 391. (86) Robin, M. B.; Day, P., Ad. Inorg. Chem. Radiochem (1967) 10, 247. (87) Creutz, C.; Taube, H., J. Am. Chem. Soc. (1969) 91, 3988; ibid (1973) 95, 1086. (88) Cowan, D. 0.; Kaufman, F., J. Am. Chem. Soc. (1970) 92, 219. (89) Ford, P.; Rudd, de F. P.; Gander, R.; Taube, H., J. Am. Chem. Soc. (1968) 90, 1187. (90) Mulliken, R. S.; Person, W. B., Molecular Complexes; Wiley, New York (1969) Chapter 2. (91) Mayoh, B.; Day, P., J. Am. Chem. Soc. (1972) 94, 2885; Inorg. Chem. (1974) 13, 2273. (92) Tom, G. M.; Creutz, C.; Taube, H., J. Am. Chem. Soc. (1974) 96, 7828. (93) Sutton, J, E.; Sutton, P. M.; Taube, H., Inorg. Chem. (1979) 18, 1017. Sutton, J. E.; Taube, H., Inorg. Chem. (1981) 20, 3125. (94) Sutin, N., Inorganic Diochemistry, Vol. 2, G. L. Eichhorn, Ed., American Elsevier, N. Y. (1973), p611. (95) Brown, G. M.; Krentzien, H. J.: Abe, M.; Taube, H., Inorg. Chem. (1979) 18, 3374. (96) Lay, P. A.; Magnuson, R. H.; Taube, H., J. Am. Chem. Soc. (1983) 105, 2507. (97) Spectroscopic studies by J. Ferguson and co-workers (Australia National University) are in progress. (98) Sheldon, R. A.; Kochi. J., Metal Catalyzed Oxidations of Organic Compounds. Academic Press, New York, 1981. (99) Ford, P.; Wink, D.; Dibenedetto, J., p 213 ofref. 1. (100) Meyer, T. J., p 389 of ref. 1. Yüklə 292,61 Kb. Dostları ilə paylaş:
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