The ground-and excited-state properties of heterometallic [CuFe3S4](2+,+), [CdFe3S4](2+,+) and [CrFe3S4](2+,+) cubane clusters assembled in Pyrococcus furiosus ferredoxin have been investigated by the combination of EPR and variable-temperature/variable-field magnetic circular dichroism (MCD) studies. The results indicate Cd2+ incorporation into [Fe3S4](0,-) cluster fragments to yield S = 2 [CdFe3S4](2+) and S = 5/2 [CdFe3S4](+) clusters and Cu+ incorporation into [Fe3S4](+,0) cluster fragments to yield S = 1/2 [CdFe3S4](+) and S = 2 [CuFe3S4](+) clusters. This is the first report of the preparation of cubane type [CrFe3S4](2+,+) clusters, and the combination of EPR and MCD results indicates S = 0 and S = 3/2 ground states for the oxidized and reduced forms, respectively. Midpoint potentials for the [CdFe3S4](2+,+), [CrFe3S4](2+,+), and [CuFe3S4](2+,+) couples, E-m = -470 +/-15, -440 +/- 10, and +190 +/- 10 mV (vs NHE), respectively, were determined by EPR-monitored redox titrations or direct electrochemistry at a glassy carbon electrode. The trends in redox potential, ground-state spin, and electron delocalization of [MFe3S4](2+,+) clusters in P. furiosus ferredoxin are discussed as a fun ction of heterometal (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Tl).