Cubane-type clusters of formula [MO3CUS4Cl4(dmpe)(3)](PF6) (4), [MO3CUS4Br4(dmpe)(3)](PF6) (5), and [W3CUS4-Br4(dmpe)(3)](PF6) (6) have been prepared by reacting the incomplete cuboidal trimers [Mo3S4Cl3(dmpe)(3)](PF6) (1), [Mo3S4Br3(dmpe)(3)](PF6) (2), and [W3S4Br3(dmpe)(3)](PF6) (3), respectively, with CuX (X = Cl or Br) or the mononuclear copper complex [Cu(CH3CN)(4)](+) in THF. The reaction takes place without global changes in the metal oxidation states, and compounds 4-6 with a [M3CuS4](5+) core possess 16 e(-) for metal-metal bonding. X-ray structural analysis of 4-6 revealed an effective C-3 nu symmetry for the M3CU unit with the M-M distances being statistically the same for M = Mo or W. However, the M-Cu distance is 0.04 and 0.1 Angstrom longer than the M-M bond length for Mo and W, respectively. There is no significant structural rearrangement of the ligand-metal bonding in proceeding from [M3S4X3(dmpe)(3)](+) to [M3CuS4X4(dmpe)(3)](+). The cyclic voltammograms of the [Mo3CUS4] cubane clusters show one quasi-reversible reduction process at E-1/2 = -0.31 V for 4 and at E-1/2 = -0.23 V for 5 and one irreversible reduction at -0.69 and -0.58 V for 4 and 5, respectively. The tungsten cluster 6 shows a unique quasi-reversible reduction wave at E-1/2 = -0.71 V. The incorporation of copper into the incomplete [M3S4] cuboidal complexes produces a decrease of the reduction potential for both molybdenum and tungsten. Absorption spectra of 1-6 are broadly similar; replacing Mo by W in proceeding from 2 to 3 or from 5 to 6 and replacing Br by Cl in proceeding from 2 to 1 or from 5 to 4 results in a blue shift of the three UV-visible absorption bands. All six clusters exhibit optical limiting, as measured by the Z-scan technique at 523 nm using 40 ns pulses. The power-limiting mechanism remains obscure, but under the conditions employed, threshold-limiting fluence decreases on replacing W by Mo on proceeding from 3 to 2 or 6 to 5 and on proceeding from tetranuclear cluster (4-6) to trinuclear precursor (1-3, respectively). For all six clusters, values of the excited-state cross section aff are larger than those of the corresponding ground-state cross section sigma (0); i.e., all clusters are efficient optical limiters.