Niobium IV Fluoride

If niobium(V) fluoride is purchased from commercial vendors, it must be purified by sublimation before use. Charges of niobium(V) fluoride (5-50 g. in a platinum crucible) are heated in 

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In a typical experiment, 4.63 g. (0.025 mole) of niobium(V) fluoride is mixed with 0.348 g. (0.012 mole) of -200-mesh silicon and loaded into the reactor inside a glove box under dry nitrogen. The assembled reactor is connected to a helium tank and repeatedly pressurized and depressurized to rid the system of air. The entire system is pressurized to 50 p.s.i., the Monel reactor valve is closed, and the reaction chamber is placed in a vertical furnace at 350°C. The pressure in the external lines is left at 50 p.s.i. to prevent entry of air. 

The loaded reactor is heated for 24 hours, then removed from the furnace, and cooled to room temperature. The system is depressurized and the silicon tetrafluoride expelled into a hood, or bled into a water scrubber. The system is purged several times with helium and the heating procedure repeated for an additional 4 hours, after which the silicon(IV) fluoride removal procedure is repeated as well. The solid black product is niobium(IV) fluoride, 3.752 g. or 90% of theoretical yield. Anal. Calcd. for NbF4 Nb, 55.0 F, 45.0. Found Nb, 55.1 F, 44.3. 

For larger quantities the above procedure is used with a greater number of silicon(IV) fluoride elimination steps. Fifty-gram batches have been prepared by scaling up this technique and using a larger metal-pipe reactor. 

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The compound niobium tetrafluoride is of importance because of the increased interest in reactions of metal tetrafluorides with basic ligands. Previous syntheses given for this compound are time consuming, generally cannot be scaled up to prepare larger quantities, or require expensive equipment not always readily available. In this synthesis, simple methods are described for the preparation of niobium(V) fluoride and niobium(IV) fluoride, which should make these compounds readily available to chemists for further comparison studies with other metal tetrafluorides. 

The following preparative method (of general utility) includes the reduction of niobium(V) fluoride with elements such as silicon, phosphorus, and boron,1 which yield highly volatile fluorides. Silicon reductions have been studied in greatest detail. Niobium(IV) fluoride can be prepared in good yield by the reaction described by the equation ... 

Niobium (IV) fluoride is a black, very hygroscopic powder. It reacts with moisture in the air to give a viscous brown liquid. Complete hydrolysis results in white Nb02 F. Niobium(IV) fluoride reacts vigorously with water to give a brown solution which forms a brown uncharacterized precipitate. 

The pentafluorides are made by direct fluorination of the metals or the pentachlor-ides. Both are volatile white solids (Nb mp 80°C, bp 235°C Ta mp 95°C, bp 229°C), giving colorless liquids and vapors. They have the tetranuclear structures shown in Fig. 18-B-3. Niobium(IV) fluoride is a black, nonvolatile, paramagnetic solid TaF4 is unknown. 

With the exception of Tap4, all halides of Nb(IV) and Ta(IV) are known. They are dark sohds, prepared by reducing the respective MX5 by heating with metal M or Al. Niobium(IV) fluoride is paramagnetic (d ) and isostructural with Snp4... 

Tetrahalides and Their Derivatives. All possible tetrahalides are known except TaF4. Niobium(iv) fluoride, is a black involatile, paramagnetic solid in which each Nb atom lies at the center of an octahedron. The other six tetrahalides differ from NbF4 and resemble each other in their structures and in being diamagnetic. 

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