Once the interactions leading to lone pair formation in the simple oxides PbO and SnO are understood, an explanation for the unusual structural trends of Pb(II) and Sn(II) compounds can be readily derived.
The sharing of electrons in some metal ceramics can result in structural distortions. While oxygen is good at sharing, other anions (sulphur, selenium and tellurium) aren’t so playful, making symmetric structures more favourable for those materials.
From PbO to PbS there is a direct transition from a distorted structure (litharge) to a symmetric structure (rocksalt). Oxygen has the required electronic p levels to interact effectively with Pb 6s and produce the high energy antibonding states needed to form a strong sterically active lone pair which can support a distorted structure (as illustrated in the above schematic). However, the sulphur 3p states are higher in energy and there is a decrease in the covalent interaction with Pb 6s. Not enough high energy antibonding states are produced to form a significant lone pair. Hence, the distorted structure becomes unstable relative to a symmetric structure of higher coordination. In this way PbS favors the symmetric rocksalt structure as its thermodynamically stable phase.
The 5s states of Sn are higher in energy than the 6s states of Pb and thus have better overlap with O 2p. As such, the interaction of Sn 5s with S 3p is stronger than for Pb. This means that SnS can also form a distorted structure, herzenbergite, which can be described as a direct distortion of the rocksalt structure. Indeed, SnSe also favours this herzenbergite structure. It is not until SnTe that the interactions between Sn and the anion become too weak to form a sterically active lone pair and the rocksalt structure is finally adopted.