INFLUENCE OF THE SOLVENT ON THE REDOX THERMODYNAMICS OF SOME MULTINUCLEAR COORDINATION-COMPOUNDS

The half-wave potentials, E1/2, and reaction entropies, DELTA-S-degrees, for the redox couples [Fe4S4(CP)4]z/z-1, [Fe4S4(MeCp)4]z/z-1, [Fe4S5(Cp)4]z/z-1 (all z = 1+, 2+, and 3+), and [Fe4(CO)4(Cp)4]z/z-1 (z = 0, 1+, and 2+) have been determined in a series of nonaqueous solvents with the Fc1+/0 couple as reference. The half-wave potentials of the 1+/0 couples are nearly independent of the solvent used. The solvent dependence of the half-wave potentials of the other couples can be described by the relation E1/2 = a1 + a2DN, where DN is the Gutmann donor number and a1 and a2 are parameters characteristic for the redox couple. For the iron-sulfur and iron-carbonyl cluster redox couples and for the couples [M(bpy)3]3+/2+ (M = Cr, Fe, and Co), [Co(phen)3]3+/2+, Sm3+/2+, Eu3+/2+, and Yb3+/2+ for which the data were taken from literature, the parameter a2 is found to correlate with (2z - 1)/r, where r is the radius of the solute. This result suggests that E1/2 of the reference couple Fc1+/0 itself shifts in the negative direction with increasing solvent donor number. In all but one case the DELTA-S-degrees (= DELTA-S-degrees rc(A(z)/A(z-1)) - DELTA-S-degrees rc(Fc1+/Fc)) values for the 1+/0 and 2+/1+ transitions of the [Fe4S4(CP)4]z series are equal within experimental precision to the corresponding values for the (1+/0) and (2+/1+) couples of the [Fe4S5(CP)4]z and [Fe4(CO)4(Cp)4]z series. This strongly confirms that there are no strong specific interactions between the cluster cores and the solvent. The data for the iron-sulfur and iron-carbonyl cluster compounds as well as the E1/2 and DELTA-S-degrees data for a number of mononuclear and binuclear (''mixed valence'') ruthenium compounds in acetone solution indicate that the influence of the solvent on the thermodynamic properties is brought about by the first layer of solvent molecules around the solute.