Niobium pentaiodide is the inorganic compound with the formula Nb2I10. Its name comes from the compound's empirical formula, NbI5.[1] It is a diamagnetic, yellow solid that hydrolyses readily. The compound adopts an edge-shared bioctahedral structure, which means that two NbI5 units are joined by a pair of iodide bridges. There is no bond between the Nb centres.[2] Niobium(V) chloride, niobium(V) bromide, tantalum(V) chloride, tantalum(V) bromide, and tantalum(V) iodide, all share this structural motif.
Niobium pentaiodide forms from the reaction of niobium with iodine:
2 Nb + 5 I2 → 2 NbI5The method used for the preparation of tantalum(V) iodide using aluminium triiodide fails to produce pure pentaiodide.[3]
Niobium(V) iodide forms of dark, brassy, extremely moisture-sensitive needles or flakes. Its crystallises in the monoclinic crystal system with space group P21/c (space group no. 14), a = 1058 pm, b = 658 pm, c = 1388 pm, β = 109.14°. The crystal structure consists of zigzag chains of corner-sharing NbI6 octahedra. Since so far only twinned crystals of this phase have been obtained, and the structure determination is uncertain.[4] If the reaction of the elements is carried out with an excess of iodine, a triclinic modification is created with the space group P1 (No. 2), a = 759.1 pm, b = 1032.2 pm, c = 697.7 pm, α = 90 .93°, β = 116.17°, γ = 109.07°, which consists of isolated molecules Nb2I10.[5] This structure is isotypic with that of triclinic niobium(V) bromide.