I would like to find out more about what makes a compound ionic and what makes one covalent. However, I felt this was too general a question so I thought that asking about the chlorides of lead would be more appropriate. I have already come up with an explanation of my own, which I have provided below. However, I would also like to hear the opinions of others.
My explanation is as follows: I consider the interaction between the atomic orbitals of the lead (II) and lead (IV) cations with the chloride ions. Firstly, we must note that their valence electronic configurations are different: lead(II) has its valence electrons in 6s orbital while the valence electrons of lead (IV) reside in the 5d orbital. On the other hand, the valence electrons of the chloride ions reside in the 3p orbital. Based on what I have read from Clayden's Organic Chemistry, I understand that atomic orbitals of similar energy interact strongly to produce strong covalent bonds. I was thinking that since the interacting orbitals of the lead (IV) cation and the chloride ions are closer in energy, a favourable covalent interaction would occur. However, in the case of lead (II), covalent bond formation is not favoured since the orbital energies are considered to be too far apart. I am not confident that this is correct. Please feel free to point out any inaccuracies.