Factors Controlling Asymmetrization of the Simplest Linear I₃^–and I₄^2–Polyiodides with Implications for the Nature of Halogen Bonding

Abstract

This paper investigates geometric and electronic features of linear I3– and I42– anions, as building blocks of larger polyiodides. Most experimental structures are quasi D∞h, although one lateral linkage is occasionally elongated with I···I separations approaching those of I···I–R– species, typical of halogen bonding (HalB). Hirshfeld surfaces from crystal data highlight solid state effects depending on the distribution of the counterions around I3– or I42– units. Corresponding experimental asymmetries have been mimicked with density functional theory calculations through different surroundings of positive point charges. The consequent deformations are interpreted in terms of the s/p rehybridizations occurring at the central I atom(s) of the populated frontier σ* wave functions. The origin is a charge-induced variation of the orbital energies at lateral iodides (electronegativity), hence by their the donor power in a nucleophilic attack. The calculations also provide energy information on I2 + I– or I2 + 2I– additions, and, in solvent, the intrinsic energy stability of I42– is for the first time validated. In the absence of positive charge perturbations, the 1– charge of a remote iodide polarizes I3– and promotes incipient electrostatic attraction, which is quickly accompanied by electron transfer with a generalized σ delocalization throughout I42–. Implicit orbital overlap supports a covalent picture, or better to say hypervalency, given the electron richness of the central atoms. Molecular electrostatic potential (MEP) surfaces are expected to show σ holes in support of the purely electrostatic HalB model, typically proposed for I···I–R– systems. However, the computed surfaces show little evidence of σ holes in the equilibrium adducts I3–, I42– and I···I–R– suggesting that HalB cannot be purely electrostatic.