Theoretical Study of Complexes of Extended Cyclopentadienyl Ligands with Zinc and Cadmium

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Abstract
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, (Science2004, 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−2M1−2M2−Dp (M1, M2 = 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−2M1−2M2−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 M1−Ep−M2 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 M1−M2 bonds. Careful electronic structure analysis gives deep insight into the nature of observed phenomena.