Factors Controlling Asymmetrization of the Simplest Linear I-3(-) and I-4(2-) 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-infinity h, although one lateral linkage is occasionally elongated with I center dot center dot center dot I separations approaching those of I center dot center dot center dot 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 sigma* 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 sigma 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 sigma holes in support of the purely electrostatic HalB model, typically proposed for I center dot center dot center dot I-R- systems. However, the computed surfaces show little evidence of sigma holes in the equilibrium adducts I-3(-), I-4(2-) and I center dot center dot center dot I-R- suggesting that HalB cannot be purely electrostatic.