Niobium Pentafluoride, NbF₅

Niobium Pentafluoride, NbF₅, is the only known compound of niobium and fluorine, and even this cannot be obtained in the free state by a wet method because of the extreme readiness with which it hydrolyses. Niobium pentoxide dissolves readily in hydrofluoric acid, but evaporation of the solution leaves a residue of the unchanged oxide. Niobium pentafluoride has been prepared synthetically by passing dry fluorine over the gently heated metal contained in a boat in a platinum tube. The product is freed from platinum tetrafluoride, a little of which is formed at the same time, by distillation in vacuo at 100° to 110° C. An alternative method consists in treating niobium pentachloride with anhydrous hydrogen fluoride in a freezing mixture and purifying by redistillation.

Niobium pentafluoride forms colourless, highly refractive prisms; density 3.2932 at 18° C. It melts at 75.5° C., and boils at 217° to 220° C. under a pressure of 760 mm. of mercury. It is extremely hygroscopic and deliquesces rapidly in air. It is reduced by hydrogen at 236° C. in contact with platinum to an unstable lower blue fluoride. Excess of concentrated alkali hydroxide or alkali carbonate solutions attack it with formation of the alkali niobate. It dissolves in toluene, paraffin, carbon bisulphide, and other organic solvents.

Niobium pentafluoride shows a strong tendency to form stable double fluorides with the fluorides of other metals. These are conveniently prepared by the action of carbonates of the metals on solutions of niobium pentoxide in a large excess of hydrofluoric acid, or by the addition of a large excess of hydrofluoric acid to solutions of the oxyfluorides of the metals. In the absence of excess of hydrofluoric acid hydrolysis takes place as usual with the formation of niobium oxytrifluoride, NbOF₃. The precipitation of these double fluorides indicates the probable existence in solution of niobium pentafluoride; stability is imparted by the formation of complex anions containing several fluorine atoms.

When viewed from the point of view of the Werner co-ordination theory, it is observed that the co-ordination number seven is frequent in the double fluorides as well as in the niobium oxyhalides. Many members of the series 2R^•F.NbF₅ or [NbF₇]R₂, and of the series 2R^•X.NbOX₃ or , are known, where R represents a metal and X is fluorine, chlorine, or bromine. The co-ordination numbers six and eight also occur, but less frequently, thus: CsF.NbF₅ or [NbF₆]Cs, and 3NH₄F.NbOF₃ or . The change in the co-ordination number can be seen in the double ammonium oxyfluorides: NH₄F.NbOF₃ or ; 2NH₄F.NbOF₃ or ; 3NH₄F.NbOF₃ or , all of which are known.

The following double fluorides of niobium have been prepared:

Cadmium Niobium Fluoride

Cadmium Niobium Fluoride, 3NbF₅.5CdF₂.5HF.28H₂O or Nb₃Cd₅F₂₅.5HF.28H₂O, is obtained in long, transparent prisms by the action of cadmium carbonate on a solution of niobic acid in concentrated hydrofluoric acid. It is insoluble in water.

Cesium Niobium Fluoride

Cesium Niobium Fluoride, CsF.NbF₅ or CsNbF₆, is obtained in fine needles by repeated crystallisation of caesium niobium oxyfluoride, Cs₂NbOF₅, from hydrofluoric acid. Another caesium niobium fluoride having the probable composition 7CsF.NbF₅ or Cs₇NbF₅ has been prepared by the action of a solution of caesium hydroxide in hydrofluoric acid on niobic acid in the same solvent.

Cobalt Niobium Fluoride

Cobalt Niobium Fluoride, 3NbF₅.5CoF₂.5HF.28H₂O or Nb₃Co₅F₂₅.

5HF.28H₂O, forms dark red, prismatic crystals, which are prepared similarly to the corresponding cadmium salt.

Copper Niobium Fluoride

Copper Niobium Fluoride, NbF₅.2CuF₂.HF.9H₂O or NbCuF₉.HF.9H₂O, forms large, dark blue crystals, which are obtained similarly to the cadmium salt.

Ferrous Niobium Fluoride

Ferrous Niobium Fluoride, 2NbF₅.3FeF₂.4HF.19H₂O or Nb₂Fe₃F₁₆.4HF.19H₂O, is obtained in greenish-yellow, thin prisms by dissolving iron and niobic acid in equivalent proportions in hydrofluoric acid.

Manganese Niobium Fluoride

Manganese Niobium Fluoride, 3NbF₅.5MnF₂.5HF.28H₂O or Nb₃Mn₅F₂₅.28H₂O, forms rose-coloured, long prisms, which are obtained similarly to the cadmium salt.

Mercury Niobium Fluoride

Mercury Niobium Fluoride, NbF₅.3HgF₂.8H₂O or NbHg₃F₁₁.8H₂O, is obtained by dissolving mercuric oxide and niobic acid in hydrofluoric acid. Concentration of the solution after removal of mercuric fluoride yields a white mass which consists of aggregates of short prismatic needles.

Nickel Niobium Fluoride

Nickel Niobium Fluoride, 2NbF₅.3NiF₂.4HF.19H₂O or Nb₂Ni₃F₁₆.4HF.19H₂O, forms thin, green needles which are prepared by dissolving equivalent quantities of niobic acid and nickel carbonate in hydrofluoric acid. Concentration of the mother-liquor gives dark green, flat prisms of the composition 3NbF₅.5NiF₂.5HF.28H₂O or Nb₃Ni₅F₂₅.5HF.28H₂O.

Potassium Niobium Fluoride

Potassium Niobium Fluoride, NbF₅.2KF or K₂NbF₇, is obtained by cooling a concentrated solution of potassium niobium oxyfluoride, NbOF₃.2KF.H₂O or K₂NbOF₅.H₂O, in hydrofluoric acid. It forms small, glistening, rhombic needles, a:b:c = 0.6682:1:0.4699. It is isomorphous with the corresponding tantalum salt but not with potassium titanium fluoride, K₂TiF₆, and hence is used in one method for the separation of titanium from niobium and tantalum. It is stable at 100° C., but loses hydrogen fluoride at considerably higher temperatures. In damp air it is slowly converted into the oxyfluoride, K₂NbOF₅. Its solution in hot water precipitates the hydrated oxyfluoride, K₂NbOF₅.H₂O, and leaves an acid solution.

The corresponding sodium niobium fluoride is unknown. Ammonium niobium fluoride cannot be prepared in the pure state because hydrolysis takes place even in the presence of excess of hydrofluoric acid. A solution of ammonium niobium oxyfluoride, 3NH₄F.NbOF₃, in hydrofluoric acid precipitates the double salt 2NH₄F.NbF₅.NH₄F.NbOF₃ or (NH₄)₃NbF₈.NbOF₃ in masses of small prisms.

Rubidium Niobium Fluoride

Rubidium Niobium Fluoride, RbNbF₆ or RbF.NbF₅, is obtained by repeated crystallisation of rubidium niobium oxyfluoride, Rb₂NbOF₅, from hydrofluoric acid. The double salt Rb₂NbF₇ or 2RbF.NbF₅ has also been reported.

Zinc Niobium Fluoride

Zinc Niobium Fluoride, 3NbF₅.5ZnF₂.5HF.28H₂O or Nb₃Zn₅F₂₅.5HF.28H₂O, is prepared in long prisms by dissolving zinc and an equivalent quantity of niobic acid in hydrofluoric acid.