One way is Polarization. When a small cation approaches a large anion it distorts the charge density towards itself thereby forming a type of bond which has directional characteristic i.e from ** (-) ----> (+) ** which is characteristic of a covalent bond.
This polarization has some rules given by **Fajan **
- Smaller the cation better distortion/polarization.
- More the amount of positive charge more is the polarization.
- or more the charge : radius ratio of cation more is the polarization.
So in short every cation has the power of polarization and therefore no bond is 100% ionic.