Insight into Spodium–π Bonding Characteristics of the MX(2)⋯π (M = Zn, Cd and Hg; X = Cl, Br and I) Complexes—A Theoretical Study

The spodium–π bonding between MX(2) (M = Zn, Cd, and Hg; X = Cl, Br, and I) acting as a Lewis acid, and C(2)H(2)/C(2)H(4) acting as a Lewis base was studied by ab initio calculations. Two types of structures of cross (T) and parallel (P) forms are obtained. For the T form, the X–M–X axis adopts a cross configuration with the molecular axis of C≡C or C=C, but both of them are parallel in the P form. NCI, AIM, and electron density shifts analyses further, indicating that the spodium–π bonding exists in the binary complexes. Spodium–π bonding exhibits a partially covalent nature characterized with a negative energy density and large interaction energy. With the increase of electronegativity of the substituents on the Lewis acid or its decrease in the Lewis base, the interaction energies increase and vice versa. The spodium–π interaction is dominated by electrostatic interaction in most complexes, whereas dispersion and electrostatic energies are responsible for the stability of the MX(2)⋯C(2)F(2) complexes. The spodium–π bonding further complements the concept of the spodium bond and provides a wider range of research on the adjustment of the strength of spodium bond.