The Effect of Single-Atom (Zn, Cd, or Hg) Replacement on the Strength and Directionality of the Spodium Bond

Abstract The spodium bond (SpB) is an attractive noncovalent interaction between the Group 12 element (Zn, Cd, or Hg) and a nucleophile (Nu). In comparison to the monovariant H-bond at R–H···Nu synthon, the SpB parameters (strength and directionality) can also be regulated by replacement of a single spodium atom (Sp = Zn, Cd, or Hg) at R–Sp···Nu. In this work we examined the effect of a single spodium atom replacement strategy on the strength and directionality of inter- or intramolecular SpB in a series of coordination and supramolecular compounds obtained from the Cambridge Structural Database. Due to their d-orbitals, Group 12 elements commonly act as π-hole donors, and there is no clear trend between the strength of Sp···Nu interactions and either the atomic polarizability or the van der Waals radius of the Sp atom. Complementary density functional theory calculations employing QTAIM and natural bond orbital analysis reveal that the bonding character of spodium interactions transitions from coordination-like interactions in Cd complexes to genuine noncovalent bonds in Hg systems, with the σ-hole mechanism serving as the primary stabilization driver.