Geometric Preferences of Tricobalt Clusters Having 47−49 Valence Electron Counts:  Deceptively Simple Cyclic Voltammetric Responses and EPR Evidence for Exchange-Coupled Dimers in Frozen Solutions^†

Oxidations and reductions of a series of 48-electron (e^-) metal clusters containing a tricobalt core have been investigated by voltammetry, electrolysis, IR spectroelectrochemistry, and EPR spectroscopy. Three isomers of Cp₃Co₃(CO)₃ have been studied. The all-CO-bridging isomer <i>C</i>₃<i>_v</i> Cp₃Co₃(μ-CO)₃ (<b>1B</b>) is stable in the 48 e^- and 49 e^- forms, but the 47 e^- cation isomerizes to a doubly-CO-bridging structure <b>1T</b>⁺. The other two isomers, Cp₃Co₃(μ₃-CO)(μ-CO)₂ (<b>1F</b>) and Cp₃Co₃(μ-CO)₂(CO) (<b>1T</b>), are present in about a 1:1 ratio near room temperature, but their solutions show only Nernstian 1 e^- cyclic voltammetry (CV) responses. IR spectroelectrochemistry and analysis of the <i>E</i>_1/2 values show that the deceptively simple CV responses arise from a rapid equilibration between isomers <b>1F</b> and <b>1T</b>, rather than from a lack of structural change accompanying charge transfer. The cluster Cp*₂Cp‘Co₃(μ₃-CO)(μ-CO)₂ (<b>3</b>) (Cp* = C₅Me₅, Cp‘ = C₅MeH₄) is shown to retain its isomeric identity through three cluster oxidation states, most likely owing to a steric preference for a face-bridging carbonyl ligand in the ring-substituted complex. The 49 e^- anions of the tricobalt complexes display EPR spectra consistent with a SOMO which is antibonding in the trimetallic plane. Additional regular hyperfine features in glassy matrices at 77 K are assigned to dimers consisting of a 49 e^- anion magnetically coupled to either a 48 e^- neutral precursor or another 49 e^- anion.