A Continuum from Halogen Bonds to Covalent Bonds: Where Do λ3 Iodanes Fit?

The intrinsic bonding nature of λ 3 -iodanes was investigated to determine where its hypervalent bonds fit along the spectrum between halogen bonding and covalent bonding. Density functional theory with an augmented Dunning valence triple zeta basis set ( ω B97X-D/aug-cc-pVTZ) coupled with vibrational spectroscopy was utilized to study a diverse set of 34 hypervalent iodine compounds. This level of theory was rationalized by comparing computational and experimental data for a small set of closely-related and well-studied iodine molecules and by a comparison with CCSD(T)/aug-cc-pVTZ results for a subset of the investigated iodine compounds. Axial bonds in λ 3 -iodanes fit between the three-center four-electron bond, as observed for the trihalide species IF 2 − and the covalent FI molecule. The equatorial bonds in λ 3 -iodanes are of a covalent nature. We explored how the equatorial ligand and axial substituents affect the chemical properties of λ 3 -iodanes by analyzing natural bond orbital charges, local vibrational modes, the covalent/electrostatic character, and the three-center four-electron bonding character. In summary, our results show for the first time that there is a smooth transition between halogen bonding → 3c–4e bonding in trihalides → 3c–4e bonding in hypervalent iodine compounds → covalent bonding, opening a manifold of new avenues for the design of hypervalent iodine compounds with specific properties.