I need to get a presentation of my PhD research ready for my first group meeting, so I have been updating some old pictures now that I am more fluent in the workings of Macintosh (a welcome transition from years of Windows). Below is an animation which illustrates the main orbital interactions which result in PbO (and likewise SnO) adopting a distorted crystal structure (litharge).
Covalent interactions (sharing of electrons) in metal oxides is more important than previously considered and can explain some unusual properties of these materials.
Surprisingly, considering some of their useful properties, the subject of lone pairs in the solid state hasn’t really been updated since the intra-atomic hybridization models of L. E. Orgel and co-workers back in the 1950’s. Their model is based on direct mixing between the cation s and p states. However, our work has shown that interaction with anion p states of appropriate energy is required to bridge the energy gap between the cation s and p which are two far apart in energy to couple directly. This can go on to explain the unusual structural trends observed in these materials that could not be explained through previous hypothesis.
The strongest covalent interaction in PbO is between the anion p and cation s states. This results in a bonding interaction at the bottom of the valence band (majority Pb 6s states). There is a corresponding filled anti-bonding combination at the top of the valence band (majority O 2p states). Pb 6p can further interact and help stabilize these antibonding states only if the coordination environment of the Pb atoms is non-centrosymmetric. This is the driving force behind PbO (and likewise SnO) adopting a distorted structure. It is the coupling of Pb 6p with the antibonding O 2p/Pb 6s states that results in the observed ‘lone pair’ in the electronic distribution of the metal cation.