Preparation transition metal borides

This method is the simplest and cheapest for making borides. It is used to prepare transition-metal borides and alkaline-earth metal hexaborides. 

In this method " - the melt eontains boric oxide and the metal oxide in a suitable electrolyte, usually an alkali or alkaline-earth halide or fluoroborate. The cell is operated at 700-1000 C depending on electrolyte composition. To limit corrosion, the container serving as cathode is made of mild steel or of the metal whose boride is sought. The anode is graphite or Fe. Numerous borides are prepared in this way, e.g., alkaline-earth and rare-earth hexaborides " and transition-metal borides, e.g, TiBj NijB, NiB and TaB... 

Recently three discrete transition metal borides have been prepared and structurally characterized. These metal-rich metallaboranes con-... 

This method is used extensively in the laboratory because it is particularly suitable for preparing borides of rare or expensive metals, e.g., the transition-metal-rich borides CrB, Cr3B4, CrB2 (except Cr3B2 and Cr4B), the diborides ScB2, TiB2 the rare-earth hexaborides, dodecaborides and MB -type borides. 

Hydroborate Reduction. Lithium or sodium tetrahydroborate and diborane can be used for reduction of metal ions, especially light transition metal ions, to produce colloidal metals. For example, colloidal copper protected by polymer was prepared by reduction of copper(II) sulfate by a large excess of sodium tetrahydroborate in the presence of PVP or other polymers (12). A similar procedure for nickel(III) chloride produced nickel boride, not zero-valence nickel metal particles. 

Chromium nitrides have been prepared by several routes heating of chromium metal in N2, reaction of chromium borides with NH3, and heating of CrCly in gaseous NH3. The two stable nitrides have the composition Cr2N and CrN see Nitrides Transition Metal Solid-state Chemistry). At very high temperatures, both decompose into the constituent elements (CrN, > 1425 °C CryN, >700 °C). CrN is very stable chemically, while CryN dissolves in dilute acid with liberation OfH2. 

Table 6 lists the most important processes for the preparation of borides, especially those of the transition metals. 

V. V. Svistunov, Investigation of Preparing and Oxidation Processes of Transition Metals Melted Borides, Thesis, Sverdlovsk, 1975. 

---