Expansion of the Metal-Involving Noncovalent Interaction Repertoire: The Case of Pd(II) and Pt(II) Triel Bonding

Supramolecular chemistry has traditionally focused on noncovalent interactions where p-block elements act as electron acceptors via σ-hole or π-hole regions. While late transition metals in square planar geometries are typically viewed as electrophilic centers, recent evidence has shown that Pd and Pt complexes can act as nucleophiles through their filled d_z2 orbitals. This behavior has been characterized for halogen, chalcogen, and pnictogen bonds, but triel bonding involving these metals remains unexplored. In this work, we analyze the interaction between square planar Pd and Pt model compounds and various triel bond donors, including BH₃, BF₃, BCl₃, BBr₃, and an aromatic boron derivative. Using DFT calculations, we evaluate the energetic and geometric features of these assemblies, which reveal the formation of moderately strong noncovalent bonds. The nature of these interactions is further characterized using the energy decomposition analysis (EDA), quantum theory of atoms in molecules (QTAIM), and natural bond orbital (NBO) analysis, confirming the significant charge transfer from the metal d_z2 orbital to the triel acceptor. Additionally, a search of the Cambridge Structural Database (CSD) provides experimental validation for the existence of these interactions in the solid state. Our results establish the triel bond as a new member of the family of metal-involving noncovalent interactions, offering new possibilities for crystal engineering and supramolecular design.