6801
Flash Photolytic, Isotope Tracer, and Chemical Scavenging Studies of the Ultraviolet Photochemistry of Pentaamminecobalt ( 111) Chloride. Evidence for Excited-State Intermediateslr2 Gunter Caspari, R. Graham Hughes, John F. E n d i ~ o t tand , ~ Morton Z. Hoffman
Contribution from the Department of Chemistry, Boston University, Boston, Massachusetts 02215. Received March 19, 1970 Abstract: Product analyses, flash photolysis, and chemical scavenging studies have shown that the oxidized radical resulting from the photoreduction of Co(NH3)6C12+is largely nitrogeneous. Of the detected final oxidized products all of the N,O (a minor product) and some of the NP result from the oxidation of NH3 in the coordination sphere of the complex at the time of photoreduction. The oxidized radicals initially formed can be stabilized by complexation with C1-. These primary radicals do not appear to react with bulk NH4+. Precursors to the formation of radicals can oxidize methanol. Methanol scavenging of these very short-lived, reactive photochemical intermediates has shown that only 5 0 x of the Co(NH&C12+originally excited by absorption of 254-nm radiation can be scavenged. Since the scavenged species normally regenerates CO(NH~)~CP+ (by means of a nonradiative decay process), it is concluded that the reactive intermediate is most likely a metastable charge-transfer excited state of the cobalt(II1) complex. espite the many studies of the photochemistry of coordination complexes of cobalt(III), questions concerning the role of excited states in determining product distributions are still controversial. A great deal of this discussion has focused on the photochemistry of simple acidoammine complexes of the type Co(NH3),X2+(X = C1-, Br-, I-). Much of the mechanistic interest in complexes of this type is the result of the several possible photochemical reactions, photoreduction5s6
D
Co(NH&X*'
+ /IY + CO'+ + .5NH4+ + X [H 'I
(1)
or photolabilization5-s
+ h~ + H20
Co(NH3)jX2+
+ XCo(NH3)dOHgX + NH4+ Co(NH3)sOHg'+
H+
(2) (3)
(1) Support of this research by the National Science Foundation (Grant No. GP-7048) is gratefully acknowledged. (2) Preliminary reports of various aspects of this study have appeared in the following places: (a) R. G . Hughes, G . Caspari, J. F. Endicott, and M. Z. Hoffman, Abstracts, 157th National Meeting of the American Chemical Society, Minneapolis, Minn., April 1969, PHYS 134; (b) R. G. Hughes, J. F. Endicott, and M. Z. Hoffman, Abstracts, 4th Mid-Atlantic Regional Meeting of the American Chemical Society, Washington, D . C., Feb 1969; (c) R. G. Hughes, J. F. Endicott, and M. Z . Hoffman, Chem. Commun., 191 (1969). (3) Address correspondence to this author at the Department of Chemistry, Wayne State University, Detroit, Mich. 48202. (4) For recent reviews, see (a) E. L. Wehry, Quart. Rev., Chem. Soc., 21, 213 (1967); (b) V. Balzani, L. Moggi, F. Scandola, and V. Carassiti, Inorg. Chim. Acta Rec., 1, 7 (1967); ( c ) A. W. Adamson, Coord. Chem. Rer., 3, 169 (1968); (d) V. Balzani, L. Moggi, and V. Carassiti, Ber. Bunsenges. Phys. Chem., 72, 288 (1968); (e) L. Moggi, V. Balzani, and V. Carassiti, ibid., 72,293 (1968); (f) D. Valentine, Jr., Adcan. Photochem., 6 , 123 (1968); (g) A. W. Adamson, W. L. Waltz, E. Zinato, D . W. Watts, P. D. Feischauer, and R. D. Lindholm, Chem. Rec., 68, 541 (1968); (h) D . Valentine, Jr., Annu. Sura. Photochem., 1, 459 (1969); (i) J. F. Endicott, IsraelJ. Chem., 8, 209 (1970). ( 5 ) (a) A. W. Adamson, Discuss. Faradaj) Soc., 29, 163 (1960); (b) A . Vogler and A. W. Adamson, J . Phys. Chem., 74,67 (1970). (6) J. F. Endicott and M. Z. Hoffman,J. Amer. Chem. Soc., 87,3348 (1965). (7) L. Moggi, N. Sabbatini, and V. Balzani, Gazr. Chim. Ital., 97,980 (1967). (8) (a) J. F. Endicott and M. Z. Hoffman, J . Amer. Chem. SOC.,90, 4740 (1968); (b) J. F. Endicott, M . Z. Hoffman, and L. Beres, J . Phys. Chem., 74, 1021 (1970).
For most of the acidoammine complexes which have been studied, reactions 1 and 2 have been reported to predominate. The variations in their relative importance with variation of X and with variation of the wavelength of the radiation used has led Adamson to argue ~ t r o n g l y ~ ~that , g , the ~ product distributions result from reactions characteristic of the solvent-trapped radical pairs, (CO(NH&~+,X generated following absorption of radiation. This radical-pair model does not ascribe any specific role to the initial or to metastable excited states which may be generated by the absorption of radiation. Several authors have contended that the radical-pair mechanism cannot account for all the known photochemistry of cobalt(II1) c o m p l e ~ e s . ~ ~ ~ ~The ~-'~~-~ fairly general observations that the photochemically active region is the near- or middle-ultraviolet has led to the suspicion that the photochemically active species is a relatively low-energy, spectroscopically forbidden charge-transfer excited state. Some recent energytransfer experiments have been interpreted as providing cm-') chargeevidence for a low-energy (
