Factors Controlling Asymmetrization of the Simplest Linear I₃–and I₄²–Polyiodides with Implications for the Nature of Halogen Bonding

This paper investigates geometric and electronic features of linear I 3 – and I 4 2– 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 I 3 – or I 4 2– 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 I 2 + I – or I 2 + 2I – additions, and, in solvent, the intrinsic energy stability of I 4 2– is for the first time validated. In the absence of positive charge perturbations, the 1– charge of a remote iodide polarizes I 3 – and promotes incipient electrostatic attraction, which is quickly accompanied by electron transfer with a generalized σ delocalization throughout I 4 2– . 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 I 3 –, I 4 2– and I···I–R – suggesting that HalB cannot be purely electrostatic.