Polysulfide Anions. 1. Structure and Vibrational Spectra of the S₂²⁻ and S₃²⁻ Anions. Influence of the Cations on Bond Length and Angle

In a comprehensive study of the M 2 S n and M‘S n alkali (M) and alkaline-earth (M‘) polysulfides, the alkali polysulfides K 2 S 2, Na 2 S 3 ·NH 3, and K 2 S 3 have been prepared by reaction of the metal with sulfur in liquid ammonia and subsequent heat treatments. Their Raman spectra have been analyzed in relation to their known X-ray structures, and that of BaS 3 has been revisited. The structure of K 2 S 2 seems intermediate in the M 2 S 2, M‘S 2 series since it has two slightly different anions in the unit cell. As for Na 2 S 2, K 2 S 2 has two α and β phases but with a much higher transition temperature ∼310 °C. These results, and those of the literature, allow the S 2 2 - bond length to be related to the variation of its electronic structure with the cation electric field. The results obtained for the M 2 S 3 and M‘S 3 polysulfides, and those of the literature, show the existence of a low- and a high-temperature phase for K 2 S 3 as for the other M 2 S 3 compounds. The S 3 2 - anion geometry, as for S 2 2 -, is directly related to the cation electric field. The opening of the SSS angle in BaS 3 is linked to the absence of cations at short distance of both terminal sulfur atoms of the S 3 2 - anion. It is thus shown that the bond length decreases with the polarizing power of the cations. The force field calculated for these anions are related to the SS bond length. The phase transformations and the stability of these compounds is explained by the polarizing power of the cation, which drives the volume left free for the anion in the crystal.