A quantitative approach to host–guest interactions for matrix-isolated alkali-metal salts of hexafluoroanions and perchlorates

Abstract

From matrix-isolation i.r. studies it is demonstrated that heavier alkali-metal salts of a number of hexafluoroanions can be vaporised as molecular species. In all cases there is some decomposition and this is particularly serious for the non-transition elements. The i.r. spectrum of CsNbF₆ in a range of matrix gases is reported in the Nb–F stretching region. It is found that for matrices where the host–guest interaction is expected to be weak (Ne, Ar) a characteristic doubling of ν₃ of the NbF₆^– ion is observed. This is interpreted as due to the presence of a tridentate (facial) interaction between the caesium ion and the NbF₆^– anion. For matrices where the host-guest interaction is expected to be stronger (CO, N₂) a characteristic triplet is observed and this is interpreted as due to a bidentate (edge) interaction between the caesium ion and the NbF₆^– ion. In an attempt to calibrate the host–guest interaction CsClO₄ is studied in a range of matrix gases. The separation between the ‘terminal’ stretching frequency and the ‘bridge’ stretching frequency is taken as a measure of the interaction between the CsClO₄ and the matrix. This leads to the following sequence in order of decreasing frequency separation: Ne > Ar > 0₂≈ F2 > Kr > Xe > N₂ > CO. (Neon showing the greatest separation is assumed to have the smallest host-guest interaction.) When the above frequency separations for CsClO₄ are plotted against those for CsNbF₆ in the same matrix gases there is a close correlation between the two. Further, those matrices in which a doublet behaviour is observed for CsNbF₆are precisely the ones which interact weakly in terms of the above sequence (Ne, Ar, O₂, F₂, Kr); a triplet is found for matrices showing a stronger interaction(CO, N₂) For xenon (and to a lesser extent krypton) both doublet and triplet forms are present. The spectrum of CsNbF₆ in a 1:1 mixture of argon and nitrogen is strikingly similar to that in xenon.