Characteristic phase transition of [C5H5NH]SbBr4 having a hypervalent bondstudied by single crystal X-ray diffraction and H-2 NMR

A characteristic second-order phase transition (T-ir = 251 K) of pyridinium tetrabromoantimonate(III) having hypervalent bonds was studied by means of the single-crystal X-ray diffraction and H-2 NMR. The SbBr4- anion in [C5H 5NH]SbBr4 forms an infinite zigzag chain of irregular SbBr6 octahedra by sh aring two edges. This SbBr4- anion has Ct symmetry on the Sb atom perpendic ular to the infinite chain. Hence, each octahedron has one symmetric trans Br(2)-Sb-Br(2) bond and two asymmetric trans Br(1)-Sb . . . Br(Z) above T-t r. However, a structural refinement on Phase II at 110 K revealed a charact eristic deformation of one symmetric trans Br(2)-Sb-Br(Z) bond to an asymme tric one,i.e., one Sb-Br(2) bond becomes shorter by 0.1 13(2) Angstrom and the other bond located at the trans position becomes longer by 0.129(2) Ang strom. The structural refinement and the H-2 NMR on an N-deuterated analog suggest that the N-H . . . Br hydrogen bond stabilizes the asymmetric trans Br-Sb . . . Br bond.