In a comprehensive study of the M2Sn and M'S-n alkali (M) and alkaline-earth (M') polysulfides, the alkali polysulfides K2S2, Na2S3. NH3, and K2S3 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 BaS3 has been revisited. The structure of K2S2 seems intermediate in the M2S2, M'S-2 series since it has two slightly different anions in the unit cell. As for Na2S2, K2S2 has two alpha and beta phases but with a much higher transition temperature similar to 310 degrees 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 M2S3 and M'S-3 polysulfides, and those of the literature, show the existence of a low- and a high-temperature phase for K2S3 as for the other M2S3 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 BaS3 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.