A New Look at the Chalcogen Bond: π-Hole-based Chalcogen (Se, Te) Bonding Which Does Not Include a σ-Hole Interaction

The solid-state structures of co-crystals of chalcogenadiazoles (ChDAs) with planar half-lantern PtII2 and mononuclear PtII complexes exhibit short Ch⋯PtII (Ch = Se, Te) contacts that occur between a metal site and the ChDA. The structures demonstrate a unique geometric feature whereby the two deep σ-holes of ChDA are turned away from a dz2-nucleophilic PtII site. A comprehensive DFT study (PBE0-D3BJ; computational tools QTAIM, ELF, IGMH, MEP, CDF, ETS-NOCV, and SAPT), focused on the analysis of these puzzling structural features, revealed that a platinum(II) center does not interact with the deep σ-holes of ChDA but is involved in other types of interactions including, in particular, with a π-hole at the Ch site. The unusual linkage between a platinum(II) atom and ChDA is driven by the combined action of the Pt → π-hole on Ch bonding (30–41% of the total interaction energy) and other C⋯C/C⋯S π-hole interactions involving the ChDA and ligand's arenes. The effect of the combined action of these interactions is so expressed that the Ch⋯Pt interaction structurally mimics a semicoordination bond. However, significant dz2(Pt) → π*(ChDA) charge transfer allows the assignment of the Ch⋯Pt interactions to the chalcogen bonding.