The electronic structures and aromaticities for zinc sandwich, half-sandwich and zinc-zinc (Zn22+) sandwich complexes within density functional theory

The equilibrium geometries, energies, harmonic frequencies, and nucleus-independent chemical shifts of the zinc sandwich, the half-sandwich, and the zinc-zinc (Zn22+) sandwich complexes are computed by B3LYP/6-311++G(d,p) within the density functional theory. The staggered [(η5-C5H5)2Zn2] (1A1, D5d) is the most stable minimum with higher binding energy and slightly stronger aromaticity than those of the η5-C5H5− (1A1, D5h). The eclipsed [(η5-C5H5)2Zn2] (1A1, D5h) is a transition state with a small ring rotation barrier. The Zn-containing half-sandwich complexes are minima while with different stabilities and aromaticity. Particularly, the [(η5-C5H5)Zn] (2A1, C5v) has larger binding energy with aromaticity slightly weaker than η5−C5H5−, this compound features a simple while bona fide monovalent zinc molecular compound. The Zn2+ sandwich complex, [Zn(η5-C5H5)2] (1A1, D5h and D5d), with aromaticity weaker than that of the slip-sandwich complex, is a saddle point on potential energy surface. The slip-sandwich complex, (η1-C5H5)Zn(η5-C5H5) (1A1, C1), with aromaticity close to that of η5−C5H5− (1A1, D5h), is a local minimum. Both the eclipsed and the staggered [(C5(CH3)5)2Zn2](C1) are aromatic with aromaticity close to that of [(η5-C5H5)2Zn2] (D5d). According to the analysis of molecular orbitals, the Wiberg bond indices, the magnitude of charge transfer, the total nucleus-independent chemical shifts distributions, and the nucleus-independent chemical shifts contribution distributions of various bonds manifests, the stabilities of all the Zn-containing sandwich, the half-sandwich, and the Zn22+ sandwich complexes are accredited to both ionic electrostatic interactions and covalent binding, especially the ionic electrostatic interactions, between the metal center and the η5−C5H5− building blocks.