Molecular hydride is classified on the basis of the presence of the bonds and total number of electrons in their Lewis structures as:
1. Electron-deficient hydrides
2. Electron-precise hydrides
3. Electron-rich hydrides
1. An electron-deficient hydride has very less electrons, less than that required for representing its conventional Lewis structure.
E.g. \(\mathrm{BH}_3, \mathrm{AIH}_3\) etc.
They exist in diameric forms such as \(\mathrm{B}_2 \mathrm{H}_6, \mathrm{AI}_2 \mathrm{H}_6\) to they make up their deficiency.
In \(\mathrm{B}_2 \mathrm{H}_6\), there are 6 bonds in all, out of which only 4 bonds are regular 2 centered 2 electron bonds. The remaining 2 bonds are 3 centered 2 electron bonds i.e., 2 electrons are shared by 3 atoms.
2. An electron-precise hydride has a sufficient number of electrons to be represented to form covalent bond. E.g. \(\mathrm{CH}_4, \mathrm{SiH}_4\) etc.
In this compound 4 regular bonds are formed where 2 electrons are shared by 2 atoms.
3. An electron-rich hydride compounds contains excess valence electrons to form covalent bonds.
E.g. \(\mathrm{NH}_3, \mathrm{PH}_3\)
There are 3 regular bonds in all with a lone pair of electrons on the nitrogen atom.