How does the charge-to-size ratio of the beryllium ion affect its bonding?

The high charge-to-size ratio of the beryllium ion (${\text{Be}}^{2+}$) significantly influences its chemical behavior, making the ionic model less applicable to beryllium than to the heavier alkaline-earth metals.1

Because the ${\text{Be}}^{2+}$ ion is very small and carries a high positive charge, it exerts a strong polarizing effect on the electron clouds of neighboring atoms or ions. This intense polarization causes the bonds formed by beryllium to be predominantly molecular or covalent rather than ionic. Consequently, while other alkaline-earth metals typically form ionic compounds, most beryllium compounds exhibit covalent character. This is further evidenced by the fact that many beryllium complexes can be extracted from aqueous solutions into organic solvents, a property characteristic of molecular substances rather than ionic ones.1