Some Aspects of the Chemistry of Samarium (II) Compounds

Although the normal oxidation state of the rare earth elements is the tripositive one, several of them are capable of exhibiting either a lower or a higher valence of variable degrees of stability. Of particular interest in this investigation was the divalent state exhibited by samarium. It is well known that half-filled electronic shells tend to be rather stable. Divalent europium with electronic configuration of Xe 4f7* forms quite stable compounds which are slowly oxidized by water. Another stable rare earth divalent ion is Yb++ where the 4f shell is complete, (Xe 4f14), divalent samarium ion, on the other hand, only approaches a half filled shell (Xe 4f6 ) and is consequently extremely susceptible to oxidation. The first known samarium (II) compound was the chloride prepared by Matignon in 1906 (1). Later investigators prepared the bromide (2) and the iodide (3). All of these compounds were obtained by hydrogen or ammonia reduction of the corresponding trivalent halides at elevated temperatures. The iodide can likewise be prepared by the thermal decomposition of the SmI3 (3). Several of the divalent samarium compounds have been prepared by metathetical reactions in aqueous solutions; the sulfate, the chromate and the phosphate by Jantsch and Skalla (3) and the fluoride, the citrate and the carbonate by Clifford and Beachell (4). Of all these compounds only the sulfate could be isolated and analyzed. Several investigators have reported a green colored samarium (II) compound (4, 5) which was, presumably, either a carbonate or a hydroxide. The unusual color is rather interesting since all other compounds of divalent samarium are red