ELECTRON-EXCHANGE KINETICS IN A TETRAHEDRALLY COORDINATED COPPER(II) (I) COUPLE/

The four-coordinate anion Cu-I(dpym)(2)(-) (Hdpym = ,5'-tetramethyl-4, 4'-dicarboethoxydipyrro-methene) can be prepared in solution in aceton e either by electrochemical reduction of the known tetrahedral complex Cu-II(dpym)(2)(0) (E(0) = -290 mV vs. SCE) or by the quantitative rea ction of 2Hdpym with Cu(CH3CN)(4)(+) in the absence of O-2. The latter reaction does not go to completion in solvents that bind relatively s trongly to Cu-I or that are poor proton accepters. Ligand exchange bet ween Cu-I(dpym)(2)(-) and excess Hdpym in acetone is ''fast'' in the H -1 NMR timeframe, with k(1) = 1.4 x 10(7) L mol(-1) s(-1) at 298 K (fi rst order in each reactant), Delta H-1(double dagger) = 3.4 +/- 0.6 kJ mol(-1), and Delta S-1(double dagger) = -97 +/- 3 J K-1 mol(-1). In t he absence of excess Hdpym, dissociation of Cu-I(dpym)(2)(-) in aceton e remains negligible. Homogeneous electron exchange between Cu-I(dpym) (2)(-) and Cu-II(dpym)(2)(0) in acetone falls in the ''slow'' H-1 NMR timeframe, with k(ex) = 5.9 x 10(3) L mol(-1) s(-1), Delta H-ex(double dagger) = 48.5 +/- 3.0 kJ mol(-1), and Delta S-ex(double dagger) = -1 0 +/- 10 J K-1 mol(-1), at ionic strength I approximate to 0.007 mol L (-1) and 298 K, while for the same self-exchange on a Pt electrode the heterogeneous rate constant k(el) = 0.16 +/- 0.04 cm s(-1) at I appro ximate to 0.1 mol L(-1) and 298 K, according to AC voltammetry. These values of k(ex) and k(el) are of the order expected for Cu-II/I couple s in which no significant change in coordination number or geometry ac companies electron transfer.