Unraveling the Nature of Ch–S Interactions in Tellurium(II) and Selenium(II) Dithiocarbamates: A Benchmark Study for Distinguishing Coordination Bonds from Intramolecular Chalcogen Bonds

This study presents the first systematic quantum chemical investigation establishing benchmark criteria for distinguishing coordination bonds from intramolecular chalcogen bonds in heavy chalcogen compounds. Through experimental and computational analysis of the tellurium(II) and selenium(II) dithiocarbamate complexes [Te(S₂CNEt₂)₂] (<b>1</b>), [Se(S₂CNEt₂)₂] (<b>2</b>), and [Se(S₂CN(CH₂)₄)₂] (<b>3</b>), we demonstrate that anisobidentate coordination arises from a bonding continuum combining covalent coordination and noncovalent chalcogen interactions. Using combined QTAIM, ELF, and RDG analysis, we established quantitative criteria for bond discrimination and discovered a diagnostic RDG method where coordination bonds appear without characteristic contours, while chalcogen bonds display blue regions at bond critical points. This work reinterprets secondary interactions as authentic intramolecular chalcogen bonds with LP(S)→σ*(Ch-S) charge transfer energies up to 39.7 kcal/mol. Results show shorter Ch-S bonds exhibit coordination (Te) and covalent (Se) characteristics, while longer contacts represent intramolecular chalcogen bonds with σ-hole signatures. Comparative analysis with the nickel(II) complex [Ni(S₂CNEt₂)₂], where the ligands are isobidentate, validates covalent nature of the Ni-S bonds through different electron densities (ρ < 0.045 au for chalcogen bonds vs 0.0875 au for the coordination bonds). The methodology provides a framework for characterizing bonding regimes in heavy chalcogen compounds.