Bonding in tetrahedral Cu4(mu3-X)4L4 copper(I) clusters: a DFT investigation

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.