Structural and Raman spectroscopic studies as complementary tools in elucidating the nature of the bonding in polyiodides and in donor-I-2 adducts

A large number of examples of polyiodides and donor-I-2, adducts whose stru ctures and Raman spectra are reasonably well-established, have been collect ed. An empirical method based on structural and Raman data, as complementar y tools in elucidating the nature of the iodine moiety, is proposed to remo ve the arbitrariness in considering discrete/non discrete entities and to p rovide a unified picture of the bonding in these species. Accordingly polyi odides higher than I-3(-) do not exist as discrete entities but are formed by a combination of I- and/or I-3(-) with I-2 through donor-acceptor intera ctions. The I-3(-) moiety represents a crossover, its structural and vibrat ional features show that it can exist as an I-3(-) entity tin accordance wi th the existence of a covalent bond interaction describable by means of a d elocalized three-center four-electron, 3c-4e, bond or I- .I-2 donor-accepto r adduct. The formal I-2-adducts of donors (D) show structural and Raman fe atures which can be explained considering three types of adducts: D.I-2 D-I -I (covalent bond interaction describable by means of a delocalized 3c-4e b ond by analogy with the I-3(-) entity case) and (D-I)(+). I-, which can be considered as stages along the reaction coordinate between the separated D and I-2 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 attri butes in the literature need corrections has been pointed out in regard to the instability inherent in these materials since the decomposition of thes e species causes spurious peaks to appear in the Raman spectrum. (C) 1999 E lsevier Science S.A. All rights reserved.