Charge-transfer complexes. Part VI. Solvent shifts of the charge-transfer bands of complexes involving organic ions as donors and acceptors

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Abstract
The energy of the charge-transfer band maximum, ECT, has been measured for a number of complexes in which one or both components are ions, in a number of solvents having a wide range of polarity. Solvent shifts of ECT have been compared with the empirical measures of solvent polarity, ET and Z, and also with the changes in ECT for similar complexes in which the iodide ion is a component. Complexes examined were those with the organic cations tropylium, N-methylacridinium, and N-methylquinolinium as electron acceptors, and the organic anion penta-methoxycarbonylcyclopentadienylide as an electron donor, with both ions and neutral molecules as the second component. Compared with complexes in which the iodide ion is a component, the charge-transfer bands of complexes of the organic ions were notably less solvent sensitive, this insensitivity being particularly marked in the ion-neutral molecule complexes. This is attributed to delocalisation of charge on these ions lowering the charge density. The solvent organisation around such an ion is then much less pronounced than that around an ion like iodide, and is little different from that to be expected around a similar uncharged molecule. A small group of solvents caused anomalous shifts of the charge-transfer band, and this is attributed to specific interaction between these solvents and the components of the complex, although the exact nature of such interaction is not clear.

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