The Boranes. Electron-deficient Substances

The reaction of magnesium boride, Mg3B2, with water would be expected from simple valence theory to result in the production of molecules of boron trihydride, BH3. Instead, the substance diborane, B2H6, is produced  

Many other boranes are known those that have been the most thoroughly investigated have formulas B4Hio, B5H9, B5H10, and B10H14. They have found some use as rocket fuels. 

B10H14 and some other borane molecules have also been found to have structures based on the icosahedron, with some of the corners not occupied by boron atoms, and with some bridging hydrogen atoms, as in diborane. The B12 icosahedron is also present in elementary boron and in the hard substance B4C. 

These substances can be described as electron-deficient substances. Electron-deficient substances are substances in which some or all of the atoms have more stable orbitals than electrons in the valence shell. The boron atom has four orbitals in its valence shell, and three valence electrons. 

A characteristic feature of the structure of most electron-deficient substances is that the atoms have ligancy that is not only greater than the number of valence electrons but is even greater than the number of 

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