Niobium (III) chloride is a green-black; crystallizes in crusts, rods, or plates. Under sufficiently high NbCl5 pressure and in the absence of air, NbCl3 is stable at 800℃. It disproportionates to NbCl5 and Nb in a temperature gradient. Only slightly air-sensitive at room temperature. Insoluble in H2O, dilute acids and dilute alkali. Attacked by oxidizing agents at varying rates depending on the concentration and the temperature. Insoluble in organic solvents, even in ethanol.
High-surface Nb metal (e.g., foil) and a slight excess of NbCl5 are placed in an evacuated reactor tube which is then sealed. The sealed horizontal reactor is heated for three days in a temperature gradient such that the end of the tube containing Nb is at 390℃, while the remainder of the tube is at 355℃. The contents are thus converted to NbCl3, which, as a result of the reversible equilibrium NbCl3 (solid) + NbCl5 (gas) = 2NbCl4 (gas), is transported into the 355℃ zone where it deposits as crystals. It can then be resublimed by reversing the temperature gradient. At the end of the procedure, only the part of the tube containing the NbCl3 is heated for a few minutes to 390°C while keeping the other end at 20℃, thus driving the NbCl5 to the cold end. The tube is allowed to cool and is then opened under anhydrous conditions.
