Question

What do you understand by (i) electron-deficient, (ii) electron-precise, and (iii) electronrich compounds of hydrogen? Provide justification with suitable examples. 

Answer 1

Molecular hydrides are classified on the basis of the presence of the total number of electrons and bonds in their Lewis structures as: 

1. Electron-deficient hydrides 

2. Electron-precise hydrides 

3. Electron-rich hydrides 

An electron-deficient hydride has very few electrons, less than that required for representing its conventional Lewis structure e.g. diborane (B₂H₆). In B₂H₆, there are six bonds in all, out of which only four bonds are regular two centered-two electron bonds. 

The remaining two bonds are three centered-two electron bonds i.e., two electrons are shared by three atoms. Hence, its conventional Lewis structure cannot be drawn. 

An electron-precise hydride has a sufficient number of electrons to be represented by its conventional Lewis structure e.g. CH₄. The Lewis structure can be written as: 

Four regular bonds are formed where two electrons are shared by two atoms. An electron-rich hydride contains excess electrons as lone pairs e.g. NH₃.

There are three regular bonds in all with a lone pair of electrons on the nitrogen atom.