1-Iodoacetylenes. Part 2. Formation constants of their complexes with Lewis bases

Abstract

Formation constants of the complexes of 1-iodoacetylenes (1)–(8) with Lewis bases (9)–(15) have been measured in solution by i.r. spectrophotometry. The stoicheiometry of the complexes, the influence of the solvent on the equilibrium position, the existence of linear free energy relationships in the series of iodinated Lewis acids RI, where R = I, Br, Cl, CN, and CCX, and the relation between i.r. frequency shifts and stability constants are discussed. With any electron donor, 1-iodoacetylenes form less stable complexes than those formed by iodine cyanide. With hard bases, iodocyanoacetylene (8) and ethyl iodopropiolate (7) give complexes which are, respectively, more stable than and as stable as those with iodine; however, iodine complexes with soft bases are more stable. This is rationalized, in terms of empirical acidity scales, by the necessity to correlate the thermodynamic and spectroscopic properties of the RI complexes by a double scale equation.