Hydrogen, elemental boron halides

Except for trimethylboron, which is a gas at room temperature, the trialkylborons are colorless liquids, but the triarylborons are crystalline. Unlike the alkyl derivatives of other elements of the Third Group, alkylborons are monomeric they are unaffected by water but react with halogens and hydrogen halides, yielding organic boron halides. 

The hydrogen reduction of the halides of nonmetallic elements, such as silicon (Reaction 8) and boron, is an essential process in the production of semiconductors and high-strength fibers. 

It is easy to reduce anhydrous rare-earth halides to the metal by reaction of more electropositive metals such as calcium, lithium, sodium, potassium, and aluminum. Electrolytic reduction is an alternative in the production of the light lanthanide metals, including didymium, a Nd—Pr mixture. The rare-earth metals have a great affinity for oxygen, sulfur, nitrogen, carbon, silicon, boron, phosphorus, and hydrogen at elevated temperature and remove these elements from most other metals. 

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