Electron acceptor–electron donor interactions. Part XXI. Charge-transfer interactions of some of the highest-valency halides, oxyhalides, and oxides with aromatic hydrocarbons and fluorocarbons. Ball–plane interactions. Group VIA

Abstract

Interactions of the compounds chromyl chloride, molybdenum, tungsten, and uranium dioxide dichlorides, molybdenum hexafluoride, tungsten hexafluoride and hexachloride, uranium hexafluoride, and tungsten oxide tetrachloride towards aromatic hydrocarbons and fluorocarbons are examined. Colour changes are observed in most cases (with CrO₂Cl₂, MoO₂Cl₂, MoF₆, WF₆, WCl₆, WOCl₄, and UF₆), although with the exception of tungsten hexafluoride it is necessary to select carefully the appropriate unreactive donors. We attribute the absorptions principally to intermolecular charge-transfer transitions. Systems sufficiently stable for measurement show very small association constants (ca. 0–0·1 I mol^–1) in carbon tetrachloride, and in aliphatic hydrocarbon and fluorocarbon solvents; whereas temperature effects, expressed as enthalpies, show –ΔH values typically less than 200 cal mol^–1. Colours of many of the two-component mixtures vanish in the frozen solids. Positions of the bands indicate an order Cr > Mo > W of decreasing electron affinities for comparable molecular species, and the materials include the strongest acceptors of this series of papers. Absorptions of molybdenum and tungsten hexafluorides and other compounds change according to the ‘inert’ solvent used, and the differences are ascribed to intermolecular solvent (e.g., cyclohexane) to acceptor transitions.