The electron-transfer properties of the dinuclear complex (fulvalenediyl)Co 2 (Et 2 C 2 B 4 H 4 ) 2, 4, have been studied by electrochemical and spectroscopic methods. This d 6 d 6 complex undergoes two reversible reductions ( E 1/2 = −2.06 and −2.52 V vs ferrocene) separated by 460 mV, indicative of a strong metal−metal interaction. ESR and optical (near-IR) spectroscopy of the ions 4 - and 4 2 -, while suggestive of significant Co−Co interactions, are ambiguous regarding the mixed-valent classification of the monoanion. IR spectra in the ν(BH) region show that the two carborane ligands in 4 - are equivalent, establishing that the monoanion is an intrinsically delocalized (class III) mixed-valent species. The oxidation of 4 proceeds in two anodic steps, only the first of which ( E 1/2 = 1.11 V) is reversible. The monocation 4 + decomposed rapidly, eluding spectroscopic characterization, but the small potential separation of the two oxidations of 4 (250 mV) suggests that 4 + is a valence-trapped species, in concert with other d 5 d 6 systems. The BH stretching frequencies in the 2450−2650 cm - 1 range are quite sensitive to metal charge and may be used to diagnose oxidation state changes in metal−carborane complexes. In comparing d 6 d 7 complexes with d 5 d 6 complexes, metal−metal interactions across a fulvalenediyl spacer appear to be significantly greater for the electronic configuration with the higher electron count. The oxidation and reduction of the mononuclear d 6 analogue CpCo(Et 2 C 2 B 4 H 4 ), 5, was studied for comparison purposes.
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Chin et al. (1998) studied this question.
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