On the coordinating properties of some solvents. A vibrational spectroscopic study of mercury(II) halides and antimony(V) chloride in solution; new concepts for Lewis basicity scales of solvents

Abstract

Raman and infrared spectra of the stretching vibrational frequencies of mercury(II) halides in solvents with widely different solvating abilities, have been recorded and combined with literature data. The frequencies decrease as the interaction of the solvent with the mercury atom in the HgX 2 entity increases. Using published data from structure determinations by X-ray diffraction in solutions and crystals, an empirical correlation of the XHgX angle and the frequency shift is obtained. An empirical scale ranking the donor strength towards a soft acceptor is proposed for more than sixty solvents with widely varying solvating properties. The numerical donor strength D S values have been obtained as the decrease in the symmetric stretching vibration frequency of the HgBr 2 molecule between the gas phase and solution. This D S scale is compared with some previously proposed scales, determined with the use of hard or borderline acceptors. The most well-known of these, the donor number D N scale based on enthalpy data of the adduct formation SbCl 5·L (L = solvent molecule) in 1,2-dichloroethane is also compared with Raman measurements of the SbCl stretching frequencies of the SbCl 5·L adducts in this solvent. The dependence of the measured donor strength of the solvent molecules on the properties of the acceptor and on the method used for the donor classification is discussed. An additional donor strength scale D H for hard acceptors is derived for 24 solvents from published data. The scale is based on Gibb's free energy of transfer of the sodium ion from a solvent to a reference solvent (1,2-dichloroethane). There is hardly any correlation between the soft D S and the hard D H scales, while the D S and D N scales show a fair agreement for solvents with hard donor atoms.