Be and Al compounds are largely covalent because of high charge density leading to high polarizing power of the cations.
Polarization power of a cation
An ionic bond is formed when the degree of polarisation is very low, while a covalent bond is formed when the degree of polarisation is very strong. A cation’s ability to distort an anion is described as its polarization power, and its polarizability is defined as the anion’s tendency to become polarized by the cation. The capacity of a cation to pull the electron cloud toward it can be described as polarizing power.