Magnesium boride

As we shall see later, borides (as well as oxides, nitrides, carbides, etc.) react with water to produce a hydrogen compound of the nonmetal. Thus, the reaction of magnesium boride with water might be expected to produce BH3, borane, but instead the product is B2ff6, diborane (m.p. -165.5 °C, b.p. -92.5 °C). This interesting covalent hydride has the structure... 

Boron forms one or more borides when it reacts with most metals. For example, the reaction between magnesium and boron produces magnesium boride, Mg3B2. 

There are many compounds that contain boron and hydrogen, and they are known collectively as the boron hydrides. Six boron hydrides were prepared by Alfred Stock in 1910-1930 by the addition of hydrochloric acid to magnesium boride that was produced in small amounts when B203 was reduced with magnesium. 

The earliest syntheses of boron hydrides were grossly inefficient, giving mixtures of products of A-5% total yields. These procedures were based upon the reaction of magnesium boride with hydrochloric acid. Only much later were yields of 11% obtained when 8N phosphoric acid was used (2). 

Dlborane(6) was obtained from the thermal decomposition of B4H10 (2). In view of the restricted amounts of products obtained by the use of magnesium boride in boron hydride syntheses, development of their chemistry proceeded slowly even after techniques had been acquired for handling these materials. 

Borides Carbon boride, CB6. and silicon borides SiB3 and SiB6 are hard, crystalline solids, produced in ihe electric furnace magnesium boride, Mgi B2, brown solid, by reaction of boron oxide and magnesium powder ignited, forms boron hydrides with HC1 calcium boride, Ca3 B2, forms boron hydrides and hydrogen gas with IIC1. 

When magnesium is heated with boron, the product is magnesium boride, MgB2 (sometimes written as Mg3B2). The hydrolysis of the compound produces Mg(OH)2 and a boron hydride. Instead of the expected product of BH3, borane, the product is B2H6, diborane. 

All the hydrides of boron, except B9H15 recently isolated by Schaeffer, were discovered by Stock (1914-20). The starting material was the gaseous product of the reaction between magnesium boride and dilute hydrochloric acid. His remarkable success was due to a vacuum technique which he developed for handling compounds sensitive to oxygen and moisture, such as the hydrides of boron and silicon. Monomeric BH3 was not found because there are insufficient electrons to stabilise the bonding in such a compound, as it is stabilised in the monomeric boron halides. 

Lithium heptapotassium di(tetrasihcide), 4676 Lithium tripotassium tetrasilicide, 4675 Magnesium boride, 0168... 

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