Structural and Raman spectroscopic studies as complementary tools in elucidating the nature of the bonding in polyiodides and in donor-I2 adducts

A large number of examples of polyiodides and donor-I2 adducts whose structures and Raman spectra are reasonably well-established, have been collected. An empirical method based on structural and Raman data, as complementary tools in elucidating the nature of the iodine moiety, is proposed to remove the arbitrariness in considering discrete/non discrete entities and to provide a unified picture of the bonding in these species. Accordingly polyiodides higher than I3− do not exist as discrete entities but are formed by a combination of I− and/or I3− with I2 through donor–acceptor interactions. The I3− moiety represents a crossover, its structural and vibrational features show that it can exist as an I3− entity (in accordance with the existence of a covalent bond interaction describable by means of a delocalized three-center four-electron, 3c-4e, bond) or I−·I2 donor–acceptor adduct. The formal I2-adducts of donors (D) show structural and Raman features which can be explained considering three types of adducts: D·I2, D–I–I (covalent bond interaction describable by means of a delocalized 3c-4e bond by analogy with the I3− entity case) and (D–I)+·I−, which can be considered as stages along the reaction coordinate between the separated D and I2 and the salt [(D–I)+]I−. The precautions to be taken in studying these species have been cited. The possibility that some of the Raman attributes in the literature need corrections has been pointed out in regard to the instability inherent in these materials since the decomposition of these species causes spurious peaks to appear in the Raman spectrum.