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Contrasting Photochemical Processes in Azido- and IsothiocyanatopentaamminecobaIt( 111) Complexes. Implications for Excited State Processes1 Guillermo J. Ferraudi, John F. Endicott,* and John R. Barber Contributionfrom the Department of Chemistry, Wayne State University. Detroit, Michigan 48202. November 20, 1973
Abstract: Contrasts in the photoreactivity of C O ( N H ~ ) ~ and N ~ Co(NH3)sNCS2+ ~+ have been explored in fluid solutions using irradiations in the range 580 3 X 3 214 nm. The predominant product of all irradiations of the azido complex has been established to be C O ( N H ~ ) ~ O H ~but N ~the ~ + ammonia , aquation process was found to be much less important for Co(NH,)5NCS2+. In a similar manner, the photoredox modes are less efficient in the isothiocyanato than the azido complex for irradiations with X G254 nm. This contrast is attributed to a "heavy atom effect", Le., to relatively more rapid electronic relaxation and deactivating cascade of photoactive excited states in the isothiocyanato complex. For such ultraviolet irradiations, thermalization of the charge transfer excited states is the postulated mechanism for the observed pIateaus in photoredox yield with excitation energy. For deep ultraviolet excitations (X 38 kK/mol for the minimate AH", = A H f mum excitation energy required to produce a radical pair with low spin cobalt(I1) and an oxidized ammine ligand. Thus this process would have energy requirements comparable to eq 7 and such a reaction mode could be stimulated by direct e ~ c i t a t i o n We . ~ cannot rule out such a reaction sequence on the basis of evidence now available; however, the observed photoreaction stoichiometries and the insensitivity of region 111 behavior to changes in first coordination sphere ligation make eq 7 seem more plausible than eq 8. More specifically we have noted that Co(NH3)5NCS2+ appears to exhibit many features of a photodissociative system for X
