Theoretical study of complexes of extended cyclopentadienyl ligands with zinc and cadmium.

Using density functional theory, we have theoretically studied various kinds of complexes of cyclopentadienyl and dicyclopentadienyl ligands with zinc and cadmium atoms of oxidation state +1. We first find that a sandwich complex Cp-Zn-Zn-Cp that was recently identified by Resta et al, (Science 2004, 305, 1136) has a large overall binding energy (=-3.19 eV), where Cp denotes the pentamethyl cyclopentadienyl group. In addition, Cp-Zn-Zn-Cp is found to have a binding energy even larger by 0.93 eV, where Cp is a cyclopentadienyl ligand without methyl groups attached. Electronic structure analysis shows accumulation of electron density between Zn atoms, confirming the existence of Zn-Zn bond that is as strong as typical transition metal-halide bonds. In addition, our calculation suggests the possible existence of similar complexes Cp-Zn-Cd-Cp and Cp-Zn-Cd-Cp with a Zn-Cd bond not known thus far. Furthermore, study on the dimetallic complexes of dicyclopentadienyl ligands also predicts results which hold potential application to organometallic chemistry and organic synthesis: (a) Complexes involving a stiff ligand Dp can presumably exist in the form of dimerized sandwich complexes Dp-2M(1)-2M(2)-Dp (M(1), M(2) = Zn, Cd) with two metal-metal bonds. Their overall binding energies amount to -1.84 to -3.48 eV depending upon the kinds of metallic atoms, the strongest binding corresponding to dizinc complex. (b) Complexes involving more flexible ligand Ep can also form similar sandwich complexes Ep-2M(1)-2M(2)-Ep, but with much larger overall binding energies (=-4.97 to -7.09 eV). In addition, they can also exist in the form of nonsandwich complexes M(1)-Ep-M(2) involving only one ligand. Unlike most of dimetallic complexes of other transition metals, syn conformations are found to be exceptionally stable due to the formation of M(1)-M(2) bonds. Careful electronic structure analysis gives deep insight into the nature of observed phenomena.