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-GO-bridging isomer C3 upsilonCP3Co3(mu-CO)(3) (1B) is stable in the 48 e(-) and 49 e(-) forms, but the 47 e(-) cation isomerizes to a doubly-GO-bridging structure 1T(+). The other two isomers, Cp3CO3(mu(3)-CO)(mu-CO)(2) (1F) and Cp3Co3(mu-CO)(2)(CO) (1T), are present in about a I:1 ratio near room temperature, but their solutions show only Nernstian 1 e- cyclic voltammetry (CV) responses. LR spectroelectrochemistry and analysis of the E-1/2 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*Cp-2'Co-3(mu(3)-CO)(mu-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.