Why is MgO a Lewis base?

Actually, magnesium oxide (MgO) is typically considered a Lewis acid rather than a Lewis base. In the context of Lewis acid-base theory, a Lewis acid is a substance that can accept a pair of electrons, while a Lewis base is a substance that can donate a pair of electrons.

In MgO, magnesium (Mg) is the cation and oxygen (O) is the anion. The oxygen atom has two lone pairs of electrons, making it capable of donating an electron pair to form a bond. However, in the case of MgO, magnesium tends to accept electrons from other species rather than donating them. Hence, MgO acts more as a Lewis acid, accepting electron pairs from Lewis bases rather than donating them.

So, while it might be possible for MgO to exhibit some Lewis basic properties under certain conditions, it is predominantly considered a Lewis acid due to the electron-accepting behavior of the magnesium cation.