The absorption spectra of triiodides

The absorption spectra of the triiodides are of particular interest in view of the striking fact, to which attention was directed ten years ago by Professor Lowry, that the same twin maxima of intensity occur at about 3500 A. and 2900 A. respectively in the absorption spectra both of iodoform CHI3 and of potassium triiodide KI3. This similarity could not be accounted for by the conventional theories of valency, which would assign three covalent C—I bonds to the former compound, whereas the most plausible formula for the other contains an I3--ion, with the probable configuration K+ [I-I+I-]. Subsequent investigations revealed similar maxima in the spectra of α-dimethyltelluronium diiodide TeMe2I2 and of the so-called “β-diiodide” [TeMe3] TeMeI4, in those of the corresponding diethyl compounds TeEt2I2 and [TeEt3] TeEtI4, and in that of the diiodide of cyclo-telluropentane C5H10TeI2. The formulae of these compounds have been written as containing either an I--ion (in the diiodides) or covalently bound iodine (in the tetraiodides). Two bands, with a much wider separation than in CHI3 or KI3, have also been recorded in the spectrum of thallium triiodide TII3; they were used as evidence that the triiodide is neither a completely ionized thallic iodide nor a thallous periodide, but must contain at least a part of the iodine linked covalently to the metal. Finally, the original bands were again recorded in the spectra of Csl3 and CsI2Br and of the quaternary ammonium halides Qml3 and QmI2Br, in which Qm represents the p-bromophenyltrimethylammonium radical. There is strong chemical evidence that the halogen atoms in these compounds form complex ions in which one atom has a positive charge and the two others negative ones, the electropositive atom always being the heaviest halogen in the complex.