Polar [NbOCl 3 ] 2n and [NbOX 4 − ] n (X = Cl, Br) Chains in the Structures of NbOCl 3 and the Thallium‐Halogenooxoniobates Tl[NbOCl 4 ] and Tl[NbOBr 4 ] ‐ Synthesis, Crystal Structures and Optical Second Harmonic Generation

The reactions of thallium(I)halides TlCl and TlBr with Nb 2 O 5 and NbCl 5 , and NbBr 5 , respectively, under the conditions for chemical vapour transport in closed evacuated ampoules in a temperature gradient from 350 °C to 300 °C give yellow Tl[NbOCl 4 ] and red Tl[NbOBr 4 ] in 50 % yield. The crystal structures (Tl[NbOCl 4 ]: orthorhombic, Pbca , a = 1259.9(1), b = 791.9(1), c = 1506.9(1) pm; Tl[NbOBr 4 ]: monoclinic, C 2, a = 1304.9(3), b = 402.7(3), c = 770.6(3) pm, β = 108.150(3)) are built of linear chains of associated [NbOX 4 − ] n (X = Cl, Br) ions and of Tl + ions located between the chains. The individual square‐pyramidal [NbOX 4 − ] ions are linked by asymmetric O···Nb–O bridges. The Tl + ions are coordinated by halogen atoms in form of strongly distorted square antiprisms for X = Cl, and in form of cubes for X = Br. The two structures are not isotypic even though they are made up of analogous building units. The centrosymmetry of the Tl[NbOCl 4 ] structure causes the polar chains to run in opposite directions. The structure of Tl[NbOBr 4 ] is acentric and the polar [NbOBr 4 − ] n chains all point into the same direction. An optical second harmonic generation experiment confirmed the recently performed redetermination of the structure of NbOCl 3 (Stroebele, Meyer 2002). NbOCl 3 crystallizes acentric with analogously polar [Nb 2 O 2 Cl 6 ] n chains.