acetonitrile (distilled from CaH₂, 700 ml)
NbCl₅ (50 g, Aldrich)
aluminium powder (200 mesh, cheap stuff, 8 g)
celite (stored in an oven for days)
THF (distilled from potassium [caution] or otherwise dry, 750 ml)

Use Schlenk techniques. Dry acetonitrile (700 ml) is added to a mixture of NbCl₅ (50 g, Aldrich) and coarse aluminium powder (8 g) in a 2 l round bottom flask fitted with a stopcocked side-arm and a seriously big magnetic follower. An overhead stirrer could be used, but make sure there are no leaks. The suspension is stirred virorously. After a brief induction period the yellow solution becomes very warm and should be cooled briefly in ice water (have this on hand). Don't chill the mixture as the reation may just stop. The solution turns red. After 3 h the solution is filtered through a bed of celite on an inert gas frit. [The Al powder remaining can be pyrophoric so exercise caution in its disposal). The solvent is removed under vacuum into a liquid nitrogen cooled trap on a Schlenk line. This gives a red crystalline solid that is allegedly NbCl₄(CH₃CN)₃. Addition of THF (750 ml) followed by thorough stirring overnight gives a YELLOW (mustard) solid and a red solution. The solid is isolated on a frit (no filter aid) and washed with THF until the solution runs yellow or nearly colourless. The solid now needs to be dried in vacuo, but it contains a lot of "free" THF so exercise caution. The process will take some time and you will probably need to get in there with a spatula to break up the lumps. Once it is dry the solid is free-flowing. Yield 50 g (70%) of NbCl₄(THF)₂.

We do this on 50 g scale because that is the size of the jar from Aldrich. Manzer's prep works very well. This report amounts to practical tips for the synthesis of an under-utilised reagent in niobium chemistry. NbCl₅ tends to undergo reduction but this material is less prone. We have synthesised Cp₂NbCl₂ directly from NbCl₄(THF)₂ and NaCp! Occasionally I have smashed the frit when trying to break up the solid. Be patient. The stuff can be stored in the gloebox for years without apparent decomposition.

We have never analysed it.

L. E. Manzer, Inorg. Chem. 1977, 16, 515