ORGANOMETALLIC THERMODYNAMICS - REDOX COUPLES INVOLVING METAL METAL BONDS

Formal potentials for couples involving the oxidation or reduction of Mn2(CO)10, [Fe(eta-5-C5H5)(CO)2]2, or ([Mo-eta-5-C5H5)(CO)3]2 have been measured. For two-electron couples, such as 2[Mn(CO)5(NCCH3)]+/Mn2(CO)10 (E-degrees' = -0.30 +/- 0.03 V vs SSCE (saturated sodium chloride calomel electrode) in CH3CN, mu = 0.1) or Mn2(CO)10/2[Mn(CO)5]- (E-degrees' = -0.69 +/- 0.01 V), the values were obtained by an equilibration technique. For one-electron couples such as Mn(CO)5/[Mn(CO)5]-(E1/2 = -0.08 +/- 0.08 V), they were measured by fast-scan microelectrode cyclic voltammetry. The free energy changes for the homolytic dissociation of Mn2(CO)10 (28 +/- 4 kcal/mol), [Fe(eta-5-C5H5)(CO)2]2 (25 +/- 5 kcal/mol), and [Mo(eta-5-C5H5)(CO)3]2 (22 +/- 3 kcal/mol) in CH3CN (mu = 0.1) were calculated by combining the potentials for the one- and two-electron couples. When these quantities are included with the potentials for intermediate one-electron couples such as [Mn2(CO)10]+/0 (E1/2 = 1.50 V), it is possible to construct complete or nearly complete Latimer diagrams which interrelate the various oxidation states. An overall pattern of reactivity toward electron transfer emerges from these diagrams. Oxidation-reduction mechanisms involving dissociation, Mn2(CO)10 --> 2Mn(CO)5, followed by electron transfer are inaccessible on reasonable time scales in solution at room temperature but could be important at higher temperatures although CO loss, Mn2(CO)10 --> Mn2(CO)9 + CO, is competitive. Once formed in CH3CN, the monomers [Mo(eta-5-C5H5)(CO)3] and Mn(CO)5 are unstable with respect to disproportionation, for example, into [Mn(CO)5(NCCH3)]+ and [Mn(CO)5]-, while [Fe(eta-5-C5H5)(CO)2] is stable. Large overvoltages exist in the two-electron oxidations of the metal-metal bonds, e.g., for Mn2(CO)10 + 2CH3CN --> 2Mn-(CO)5(NCCH3)+ + 2e-, the overvoltage is + 1.80 V, and in their reductions, Mn2(CO)10 + 2e- --> 2[Mn(CO)5]-, at electrodes, or in solution. These overvoltages exist because the reactions occur by one-electron steps to or from the electrode or to or from a reagent in solution and occur through intermediates, which are thermodynamically unstable, e.g., [Fe(eta-5-C5H5)(CO)2]2+ or [Fe(eta-5-C5H5)(CO)2]2-. A related situation exists in the oxidation of the adjacent anions, to the metal-metal bonds, e.g., 2[Mn(CO)5]- --> Mn2(CO)10 + 2e-, or in the reduction of the adjacent cations, 2[Mn(CO)5(NCCH3)]+ + 2e- --> Mn2(CO)10, since these reactions occur via the unstable intermediates Mn(CO)5, Fe(eta-5-C5H5)(CO)2, or Mo(eta-5-C5H5)(CO)3.