No solid solution compounds in between the binaries: syntheses and crystal structures of Nb(Br0.62(4)Cl0.38(4))2Cl2and NbI2Cl2

Abstract The anion-mixed niobium tetrahalides Nb(Br 0.62(4) Cl 0.38(4) ) 2 Cl 2 and NbI 2 Cl 2 were obtained by heating NbBr 5 with NbCl 5 and NbI 5 with NbCl 5 , respectively, in equimolar ratios with niobium metal in evacuated, torch-sealed silica ampoules at 720 K for 3 days. The orthorhombic title compounds form as very brittle black needles and were characterized by single-crystal X-ray diffraction [space group: Immm , Z =4; a =704.27(6), b =824.13(7), c =929.64(8) pm for Nb(Br 0.62(4) Cl 0.38(4) ) 2 Cl 2 and a =753.76(6), b =829.38(7) and c =983.41(8) pm for NbI 2 Cl 2 ]. Surprisingly enough, these mixed-anionic halides are not isostructural with either NbCl 4 , NbBr 4 or NbI 4 , but crystallize isotypically with TaI 2 Cl 2 , thus being examples for differential site occupancy stabilized materials. Structural features of other niobium(IV) halides are compiled and compared to those of Nb(Br 0.62(4) Cl 0.38(4) ) 2 Cl 2 and NbI 2 Cl 2 . Except for NbF 4 , they all exhibit chains of trans -edge connected [Nb X 6 ] 2− octahedra, which allow Peierls distortions to form Nb–Nb single bonds. The packing of these chains differ, however, depending on the actual halide or mixed-halide combination.