THE SO2 STRETCHING VIBRATIONS IN SOME METAL SACCHARINATES - SPECTRA-STRUCTURE CORRELATIONS

The infrared spectra of a series of metal saccharinates (those of Na, Mg, Mn Fe, Co, Ni, Zn, Cd, Pb as well as mercury(II) saccharinate and chloromercury(II) saccharinate*) were studied in the region of the antisymmetric and symmetric SO2 stretching vibrations [nu(as)(SO2) and nu(SO2) here-after]. The results concerning the number of bands attributable to the above-mentioned vibrations, their frequencies and intensities were correlated with the crystallographically determined type of metal-to-ligand bonding, on the one hand, and the number of the structurally different SO2 groups, on the other. It was shown that, irrespective on whether the sulphonyl oxygen atoms do or do not participate in bonding to the metal atom and/or in hydrogen bonding and irrespective on the type of the metal-to-saccharin bond (from covalent to purely ionic), the frequencies of both nu(as)(SO2) and nu(s)(SO2) modes are lower for the metal saccharinates than for saccharin itself, the effect being more pronounced in the case of the nu(as)(SO2) bands. As expected, the appearance of the spectra in the SO2 stretching regions is rather strongly dependent on the values of the O-S-O angles. Thus, two rather different types of O-S-O angles exist in the structure of the covalently bonded saccharinate of mercury and of the ionic lead saccharinate (the values of these angles are 118.7 and 111.8 degrees and 120.4 and 111.8 degrees respectively) and two pairs of nu(as)(SO2) and nu(s)(SO2) bands are present in the spectra of these two compounds. On the other hand, the existence of only one type of SO2 groups in the saccharinates of Mn and ClHg or the presence of SO2 groups with very close O-S-O angles in the saccharinates of Na and Mg is manifested by the appearance of only a single pair of intense nu(as)(SO2) and nu(s)(SO2) bands in their spectra.