Synthesis and crystal structure of K3[NpO4(OH)2

Abstract

Attempts were made to isolate anhydrous compounds of [NpO4(OH)2]3− anions with heavy alkali metal cations (K, Rb, Cs) by crystallization at elevated temperatures, and the salt K3[NpO4(OH)2] was isolated and studied. Crystals of K3[NpO4(OH)2] consist of tetragonal bipyramidal [NpO4(OH)2]3− anions and K+ cations. The [NpO4(OH)2]3− anion occupies the position in the symmetry center. The Np-O distances in this anion are 1.8992(7) and 1.9100(7) A in the equatorial plane of the bipyramid and 2.3231(8) A with OH groups. The OH hydrogen atoms participate in weak H bonds [O⋯O 3.0250(11) A] linking the anions in layers [NpO4(OH)2] 3− parallel to the (010) plane. In the interlayer space, there are two crystallographically different K atoms. Their coordination number (CN) can be considered to be equal to 7 and 8. Comparison with the structures of the known compounds Na3[NpO4(OH)2] and K3[NpO4(OH)2]·2H2O was made. The structure of the latter compound was redetermined with higher accuracy. The failure of attempts to prepare the anhydrous rubidium and cesium compounds is probably due to large ionic radius of these cations, requiring high coordination number.