Q.Explain electron deficient and electron rich hydrides with one example each.

Hydrides are classed by whether the central atom has too few electrons for normal bonding (electron-deficient, e.g. B2H6) or has extra lone-pair electrons (electron-rich, e.g. NH3).

Covalent hydrides of p-block elements are classified as electron-precise, electron-deficient, or electron-rich, based on the number of valence electrons on the central atom relative to what is needed to form simple 2-electron covalent bonds to each hydrogen.

Electron-deficient hydrides:

These are hydrides in which the central atom does not have enough valence electrons to form conventional 2-electron, 2-centre covalent bonds to all the hydrogen atoms attached, and so they act as Lewis acids (electron-pair acceptors).

Example: Diborane, B2H6 (Group 13 hydride). Boron has only 3 valence electrons, insufficient to form the required number of normal bonds; B2H6 is stabilised instead by two unusual 3-centre-2-electron (banana) bonds bridging the two boron atoms through two hydrogen atoms.

Electron-rich hydrides:

These are hydrides in which the central atom has more valence electrons than are needed to form the required number of bonds to hydrogen, so it retains one or more lone pairs after bonding. They act as Lewis bases (electron-pair donors). …