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†
journal contributionposted on 29.12.1998 by Matthew P. Robben, Philip H. Rieger, William E. Geiger
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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 C3v Cp3Co3(μ-CO)3 (1B) is stable in the 48 e- and 49 e- forms, but the 47 e- cation isomerizes to a doubly-CO-bridging structure 1T+. The other two isomers, Cp3Co3(μ3-CO)(μ-CO)2 (1F) and Cp3Co3(μ-CO)2(CO) (1T), 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 E1/2 values show that the deceptively simple CV responses arise from a rapid equilibration between isomers 1F and 1T, rather than from a lack of structural change accompanying charge transfer. The cluster Cp*2Cp‘Co3(μ3-CO)(μ-CO)2 (3) (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.