The functional dependence of the rate constants for self-exchange, kex, for a series of metallocene redox couples to solvent-induced variations in the nuclear frequency factor, vn, engendered by alterations in the longitudinal solvent relaxation time, tl, are utilized to deduce values of the electronic matrix coupling element, Hn, for electron exchange. The analysis exploits the sensitivity of the kex-rL"' dependence to the degree of reaction adiabaticity and hence Hn for a given electron-exchange reaction. Six metallocene couples are examined: Cp2Co+',°, Cp2Fe+,,cl (Cp = cyclopentadienyl), the decamethyl derivatives Cp'2Co+/°a nd Cp'2Fe+/°s crutinized previously, with additional solvent-dependent kex values for (carboxymethyl)cobaltocenium-(carboxymethyl)cobaltocene [Cpe2Co+/°, e = "ester"] and (hydroxymethyl)ferrocenium-(hydroxymethyl)ferrocene [HMFc+/0], Kinetic data are examined in 15 solvents, including 11 "Debye" solvents for which it is anticipated that t>" * tl"'. Corrections to kcx for the solvent-dependent variations in the barrier height were obtained by corresponding measurements of the optical electron-transfer energies for the related binuclear complex biferrocenylacetylene, yielding "barrier-corrected" rate constants, k'sx. The k'^-rU1 dependencies, as well as the kex values in a given solvent, are markedly dependent on the redox couple. The log k 'ex-log tl"* plots for the most facile couple, Cp,2Co+/0, exhibit slopes approaching unity for smaller tl"* values. The less facile couples yield smaller slopes, diminishing in the same sequence that k'ex decreases in a given solvent: Cp'2Co+/0 > Cpc2Co+/0 > Cp2Co+/°> Cp'2Fe+/°> Cp2Fe+/0 £ HMFc+/0. These findings are consistent with Hl2 decreasing in the same order. Comparison of such rate-solvent friction dependencies with corresponding plots calculated using a combined solvent friction-electron tunneling model yields the following approximate values of the matrix coupling element for reactant "closest approach", H°n (kcal mol"1): Cp'2Co+/°, 1.0; Cpe2Co+/°, 0.5-1.0; Cp2Co+/°, 0.5; Cp'2Fe+/°, 0.2; Cp2Fe+/°, 0.1; HMFc+/°, 0.075. Reasonable concordance is seen with recent theoretical estimates of H°12 for Cp2Co+',°a nd Cp2Fe+/°. The relationship between H°n and metallocene electronic structure is briefly discussed. The analysis also enables effective solvent relaxation times for adiabatic barrier crossing in non-Debye media, including primary alcohols, to be extracted.