The crystal structures and the disorder behaviour of NO[SbCl6] and NO[TaCl6]

Despite the fact that both nitrosonium salts NO[SbCl6] and NO[TaCl6] crystallize in the same space group C2/c with Z = 4, their disorder behaviour is different. While in the antimony compound the reorientation jumps of the NO+ ions are restricted to two alignments of equal weight, the NO+ disorder of NO[TaCl6] is in principle sixfold, but with two of the alignments underrepresented. NO[SbCl6] undergoes a phase transformation to C (1) over bar at T-c = 104 K. NO[TaCl6] stays monoclinic down to 10 K. An extrapolation of the isotropic displacements against absolute zero reveals permanent components due to genuine disorder. A computer simulation is used to investigate the unusual population of the NO+ sites.