DFT calculations on Cu-4(mu(3)-X)(4)L-4 (X = H, CH3, CCH, F, Cl, Br, I; L = NH3, PH3) indicate that, regardless of its nature, X- acts essentially as a two-electron sigma-type ligand and that the covalent part of the Cu...Cu bonding depends mainly upon the a(1) component of the orbital interaction between the L4Cu44+ and X-4(4-) fragments. The first excited state corresponds to the occupation of a Cu...Cu bonding LUMO of a(1) symmetry, which is of dominant Cu(4s/4p) character when X-is an electronegative ligand, such as a halide. Consequently, this excited state is computed to exhibit Cu...Cu distances shorter than those in the ground state, in agreement with the luminescence properties of this type of compound.