Volumes of activation for electron transfer in low-spin/low-spin cationic couples in aqueous solution

Volumes of reaction Delta-V-Ag/AgCl (vs Ag/AgCl/4.0 mol L-1 KCl) and of activation Delta-V(el)double-dagger for the electrode reactions of the aqueous Co(azacapten)(3+/2+), Ru(en)(3)(3+/2+), and Co(tacn)(2)(3+/2+) couples have been measured by high-pressure cyclic and AC voltammetry. For the low-spin/low-spin Co(azacapten)(3+/2+) couple, Delta-V(el)double-dagger = -3.3 +/- 0.4 cm(3) mol(-1), whereas high-pressure NMR measurements gave a volume of activation Delta-V(ex)double dagger for the self-exchange reaction of -6.5 +/- 0.5 cm(3) mol(-1), in accordance with the "fifty-percent rule" (J. Am. Chem. Sec. 1997, 119, 7137) and with the prediction of an adaptation of the Marcus theory of intermolecular electron-transfer kinetics (Can. J. Chem. 1996, 74, 631). For the Ru(en)(3)(3+/2+) self-exchange reaction, Delta-V(ex)double-dagger was estimated indirectly as -15.1 +/- 1.7 cm(3) mol(-1) from the Co(phen)(3)(3+)/Ru(en)(3)(2+) cross reaction (Delta-V(12)double-dagger = -12.9 +/- 0.5 cm(3) mol(-1)), for which the rate constant k(12) was consistent with the Marcus cross relation. For the Fe(H2O)(6)(3+)/Ru(en)(3)(2+) cross reaction (Delta-V(12)double-dagger = -18.3 +/- 1.2 cm(3) mol(-1)), k(12) was slower than predicted from the Marcus cross relation, and consequently the estimated Delta-V(ex)double-dagger for Ru(en)(3)(3+/2+) (-18.9 +/- 2 cm(3) mol(-1)) may be less reliable. For the Ru(en)(3)(3+/2+) electrode reaction, Delta-V(el)double-dagger = -7.5 +/- 0.4 cm(3) mol(-1), again in accordance with the fifty-percent rule and, conversely, authenticating the estimated Delta-V(ex)double-dagger. The Delta-V(ex)double-dagger estimates for Ru(en)(3)(3+/2+), however, are some 10 cm(3) mol(-1) more negative than can be accommodated by the adapted Marcus theory. For the low-spin/high-spin couple Co(tacn)(2)(3+/2+) Delta-V(el)double-dagger (-5.9 +/- 0.9 cm(3) mol(-1)) is intermediate between values expected for Co-III/II clathrochelates and low-spin/high-spin tris(bidentate) chelates, although Delta-V-Ag/AgCl places this couple within the latter group.