THE LIGAND CAGE EFFECT IN VOLUMES OF ACTIVATION FOR ELECTRON-TRANSFERREACTIONS OF COBALT(III II) COMPLEXES IN AQUEOUS-SOLUTION/

The electrode reaction Co(diamsar)(3+/2+) in water at 25 degrees C, io nic strength I = 0.14 mol L(-1), is characterized by E degrees' = -0.2 4(5) V and Delta V = +17.4 +/- 0.5 cm(3) mol(-1) vs Ag/AgCl/4 mol L(-1 ) KCl, and the kinetics of the corresponding self-exchange reaction in homogeneous solution by k(ex) = 0.48 +/- 0.01 L mol(-1) s(-1), Delta H-ex(double dagger) = 50.7 +/- 0.6 kJ mol(-1), Delta S-ex(double dagge r) = -81 +/- 2 J K-1 mol(-1), and Delta V-ex(double dagger) = -10.4 +/ - 0.5 cm(3) mol(-1). For the couple Co(diamsarH(2))(5+/4+) at 25 degre es C, the corresponding parameters are E degrees' = +0.03(2) V and Del ta V = +19.5 +/- 0.8 cm(3) mol(-1) (I = 0.20 mol L(-1)) vs Ag/AgCl/4 m ol L(-1) KCl, k(ex) = 0.0149 +/- 0.0005 L mol(-1) s(-1), and Delta V-e x(double dagger) = -9.6 +/- 0.8 cm(3) mol(-1) (I = 0.39 mol L(-1)). Th e rate constants and volumes of activation are as expected for these e lectron transfer reactions on the basis of an adaptation of Marcus-Hus h theory. Comparison with data for other cation-cation self-exchange r eactions shows that low-spin/high-spin Co-III/II cage complexes behave normally in the Marcus-Hush context, despite the large change in spin multiplicity accompanying electron transfer, whereas nonencapsulated Co-III/II chelate couples deviate strongly from the norm, especially i n Delta V-ex(double dagger) values which are anomalously strongly nega tive. This is attributed to structural changes associated with a Co-II spin equilibrium preceding electron transfer in the nonencapsulated c helates-changes which are suppressed in the cage environment. The alte rnative view, that the anomalous behavior of the Co chelates stems fro m nonadiabaticity, now seems untenable. Thus, spin multiplicity change per se does not inhibit electron transfer, i.e., such reactions are n ot ''spin forbidden'', but distortions that may accompany it in electr on transfer between flexible complexes may retard the reaction and mak e a substantial negative contribution to Delta V-ex(double dagger).