Sulfur Chains Terminated by Cyano Groups

The thiocyanate group has pseudohalogen properties, and the cyanogen polysulfides may alternatively be termed sulfur thiocyanates. The behavior of the tri- and tetrasulfides towards nucleophilic reagents 82) is consistent with the thiocyanate nomenclature. 

The three lowest members of the corresponding selenium series are known. Cyanogen triselenide, or selenium diselenocyanate Se(SeCN)2, was prepared by Verneuil 219) in 1886, by treatment of potassium selenocy-anate with oxidizing agents the yellow crystals melt at 132° and are quite stable in the dark. Three analogs containing mixed sulfur-selenium chains have been described, namely diselenium dithiocyanate Se2(SCN)2 and the compound SeS(SCN)2 by Baroni 16) in 1936, and selenium dithiocyanate Se(SCN)2 by Ohlberg and vander Meulen 180) in 1953. 

The presence of unbranched sulfur chains in the cyanogen polysulfides is evident from the Raman spectra recorded by Feh r and Weber 77). The characteristic S—S stretching and S—S—S bending frequencies occur, and the CsN frequency varies with increasing chain length in a way analogous to unbranched methylene a,cc-dinitriles. 

The crystal structures of selenium dithiocyanate 181) and selenium diselenocyanate 6) have been determined by X-ray diffraction. The crystals of these compounds and of sulfur dithiocyanate 94) are isomorphous, and the three structures are accordingly analogous. The space group is Dihlt-Pnma with four molecules per unit cell, of dimensions as listed in Table I. A mirror plane of molecular symmetry is crystallographically 

The dipole moment of selenium diselenocyanate in benzene at 25° has been measured by Rogers and Gross 194). The observed value, 3.98 D, can be accounted for 194) by assuming free rotation about the Se—Se bonds, or by postulating a mixture of rotational isomers. 

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