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DFT calculations on Cu(4)(mu3-X)4L4 (X = H, CH(3), CCH, F, Cl, Br, I; L = NH(3), PH(3)) 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 a1 component of the orbital interaction between the L4Cu4(4+) and X4(4-) fragments. The first excited state corresponds to the occupation of a Cu...Cu bonding LUMO of a1 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.
Vega et al. (Thu,) studied this question.