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

Reaction of the ternary transition metal chalcogenide K4Nb2S11 with K2Se5 l eads to the formation of the new mixed sulfur selenium compound K4Nb2S8.9Se 2.1. Single crystal structure determination proves that the connectivity wi thin the molecular [Nb2S11](4-) anion remains intact but distinct sulfur po sitions are partially substituted by selenium. All S-2(2-) anions are selec tively exchanged by selenium whereas the S2- anions are not affected. The r efinement of the site occupation factors demonstrates that the two sulfur a toms in the S-2(2-) anions were non-statistically substituted by the seleni um atoms. Furthermore, the partial substitution of S by Se leads to a new s tructural 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 K4Nb2S9.5Se1.5 showing an exchang e pattern within the S-2(2-) anions being slightly found in K4Nb2S8.9Se2.1. (C) 2000 Editions scientifiques et medicales Elsevier SAS. All rights rese rved.