Chemical reactivity of the [Nb2S11]4− anion: selective substitution of sulfur by selenium on a molecular level

Reaction of the ternary transition metal chalcogenide K 4 Nb 2 S 11 with K 2 Se 5 leads to the formation of the new mixed sulfur selenium compound K 4 Nb 2 S 8.9 Se 2.1 . Single crystal structure determination proves that the connectivity within the molecular [Nb 2 S 11 ] 4− anion remains intact but distinct sulfur positions are partially substituted by selenium. All S 2 2− anions are selectively exchanged by selenium whereas the S 2− anions are not affected. The refinement of the site occupation factors demonstrates that the two sulfur atoms in the S 2 2− anions were non-statistically substituted by the selenium atoms. Furthermore, the partial substitution of S by Se leads to a new structural type for compounds with general formula A 4 M 2 Q 11 (A=alkali metals, M=Nb, Ta, Q=S, Se). The direct synthesis using a S/Se mixture yields a second compound with composition K 4 Nb 2 S 9.5 Se 1.5 showing an exchange pattern within the S 2 2− anions being slightly different from that found in K 4 Nb 2 S 8.9 Se 2.1 .