When an element requires 3 or 4 electrons to complete their octet, they cannot form ionic bond because one require a large amount of energy to completely remove an electron from ones outer shell and transfer it to the other. Ionic bond is permanent transfer of electrons. Thus, they form covalent bond.
Electronegativity difference between elements involved also plays a major role in bond formation. If electronegativity between elements is almost similar, then they will form covalent bonds whereas if difference is large, they will form ionic bond. Calcium and Carbon have greater electronegativity difference and hence it forms ionic bond with Calcium.
