STRUCTURAL CONSEQUENCES OF ELECTRON-TRANSFER REACTIONS .21. STRUCTURAL-CHANGES COUPLED TO 2-ELECTRON-TRANSFER REACTIONS - OXIDATION MECHANISM OF PSEUDO-TRIPLE-DECKER COMPLEXES OF CO AND RH

A series of four pseudo-triple-decker complexes (η5-C5R5)2M2(μ-C8H8), M = Co or Rh, R = H or Me, undergoes a chemically reversible two-electron oxidation. The neutral complexes have the two metals on either side of a tub-shaped cyclooctatetraene bridging ligand, whereas the dications have a twisted bridging C8 ring which has two planar C4 fragments. The two dicobalt complexes Cp2Co2(μ-C8H8), 1, and Cp⋆2Co2(μ-C8H8 1, 2, display quasi-reversible couples with E° = +0.10 V vs SCE, ks> 2 ⨯ 10-2 cm/s and E° =-0.22 V, ks, = 4.6 ⨯ 10-3 cm/s, respectively, in CH2C12. This conclusion is based on data from cyclic voltammetry, controlled potential coulometry, dc polarography, and rotating ring-disc voltammetry experiments. Similar experiments on the dirhodium complex 3, Cp2Rh2(μ-C8H8), show more complex voltammetric behavior. The oxidation wave for 3/32+ is over 500 mV positive of the reduction wave for 32+/3, with CV peak potentials of ca. +0.43 V for the former and-0.11 V for the latter. This couple is described by a square scheme in which the electron-transfer reactions are experimentally separable from the tub ⥩ twist isomerizations. A rate of ca. 2 s-1 was measured for the twist ⟶ tub isomerization of 3 by cyclic voltammetry, double potential step chronoamperometry, and ring-disc voltammetry. The slower isomerizations of the Rh complex are consistent with larger rearrangement energies for Rh-olefin bonds as compared with Co-olefin bonds. The two-electron oxidation wave of 3 separated into two one-electron waves, the first being reversible, in low-temperature CV experiments in CH2C12. When combined with fast-scan CV on the reduction of 32+, these experiments show that the chemically reversible couple for tub-3-2e- ⥩ twist-32+ proceeds in both directions via EEC mechanisms. This result is compared to other two-electron transfers with associated geometric changes. © 1990, American Chemical Society. All rights reserved.