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

Abstract

Some of the conclusions from the preceding papers are tabulated. In terms of equilibrium, enthalpy, and thermochromism measurements nearly all interactions qualify for the description ‘extremely weak or experimentally contact’. There is a need for further study, for example, of thermodynamic excess functions. As two major reasons why they are weak it is suggested that (1) geometrical factors impede the already feeble interactions of purely π-donors, and (2) for an acceptation orbital situated mainly on the central atom the overlap is small. If displacement of the spectra are interpreted as due to changes in molecular electron affinities of the acceptor MX_n, it appears that for a series of related compounds of the Group A elements, strongest acceptors occur towards the top of the Periodic Table, whereas the Main Groups B behave ‘atypically’ and are strongest for the heavier elements. Such trends are related by a simple thermodynamic cycle to the electron affinity of the ion Mⁿ⁺, the work of bringing up an electron to the centre against the ligand screening electrons, and a difference in enthalpy terms that includes the heats of formation.