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, D_5d) is the most stable minimum with higher binding energy and slightly stronger aromaticity than those of the η⁵-C₅H₅^- (¹A ₁, D_5h). The eclipsed [(η⁵-C ₅H₅)₂Zn₂] (¹A ₁, D_5h) 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, C_5v) 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, D_5h and D_5d), with aromaticity weaker than that of the slip-sandwich complex, is a saddle point on potential energy surface. The slip-sandwich complex, (η¹-C ₅H₅)Zn(η⁵-C₅H₅) (¹A₁, C₁), with aromaticity close to that of η⁵-C₅H₅^- (¹A ₁, D_5h), is a local minimum. Both the eclipsed and the staggered [(C₅(CH₃)₅)₂Zn ₂](C₁) are aromatic with aromaticity close to that of [(η⁵-C₅H₅)₂Zn₂] (D_5d). 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 Zn₂ ²⁺ sandwich complexes are accredited to both ionic electrostatic interactions and covalent binding, especially the ionic electrostatic interactions, between the metal center and the η⁵-C ₅H₅^- building blocks.