The chemistry of post transition metals is dominated by the group oxidation state N and a lower N-2 oxidation state, which is associated with occupation of a metal s(2) lone pair, as found in compounds of Tl(I), Pb(II) and Bi(III). The preference of these cations for non-centrosymmetric coordination environments has previously been rationalised in terms of direct hybridisation of metal s and p valence orbitals, thus lowering the internal electronic energy of the N-2 ion. This explanation in terms of an on-site second-order Jahn-Teller effect remains the contemporary textbook explanation. In this tutorial review, we review recent progress in this area, based on quantum chemical calculations and X-ray spectroscopic measurements. This recent work has led to a revised model, which highlights the important role of covalent interaction with oxygen in mediating lone pair formation for metal oxides. The role of the anion p atomic orbital in chemical bonding is key to explaining why chalcogenides display a weaker preference for structural distortions in comparison to oxides and halides. The underlying chemical interactions are responsible for the unique physicochemical properties of oxides containing lone pairs and, in particular, to their application as photocatalysts (BiVO(4)), ferroelectrics (PbTiO(3)), multi-ferroics (BiFeO(3)) and p-type semiconductors (SnO). The exploration of lone pair systems remains a viable a venue for the design of functional multi-component oxide compounds.

The geometric and electronic structures of MoO3 and MoO2 have been calculated using the generalized gradient approximation to density functional theory. The calculated cross-section weighted densities of states are compared with high-resolution X-ray photoemission spectra. There is very good agreement between the calculated structures and those determined previously by X-ray diffraction and between the computed densities of states and the present photoemission measurements. MoO2 is shown to be a metallic material, as is found experimentally, but the Fermi level sits in a distinct trough in the density of states. Satellite peaks found in core photoemission spectra of MoO2 are shown to derive from final state screening effects in this narrow band metallic material.

Bulk and surface sensitive x-ray spectroscopic techniques are applied in tandem to show that the valence band edge for In2O3 is found significantly closer to the bottom of the conduction band than expected on the basis of the widely quoted bulk band gap of 3.75 eV. First-principles theory shows that the upper valence bands of In2O3 exhibit a small dispersion and the conduction band minimum is positioned at Gamma. However, direct optical transitions give a minimal dipole intensity until 0.8 eV below the valence band maximum. The results set an upper limit on the fundamental band gap of 2.9 eV.

The bulk and surface electronic structure of In 2 O 3 has proved controversial, prompting the current combined experimental and theoretical investigation. The band gap of single-crystalline In 2 O 3 is determined as 2.93Ϯ 0.15 and 3.02Ϯ 0.15 eV for the cubic bixbyite and rhombohedral polymorphs, respectively. The valence-band density of states is investigated from x-ray photoemission spectroscopy measurements and density-functional theory calculations. These show excellent agreement, supporting the absence of any significant indirect nature of the In 2 O 3 band gap. Clear experimental evidence for an s-d coupling between In 4d and O 2s derived states is also observed. Electron accumulation, recently reported at the ͑001͒ surface of bixbyite material, is also shown to be present at the bixbyite ͑111͒ surface and the ͑0001͒ surface of rhombohedral In 2 O 3 .

Structural distortions in post-transition metal oxides are often explained in terms of the influence of sp hybrid "lone pairs." Evidence is presented here showing that this model must be revised. The electronic structures of prototypically distorted alpha-PbO and alpha-Bi2O3 have been measured by high-resolution x-ray photoemission and soft x-ray emission spectroscopies. In contrast with the expectations of the lone pair model, a high density of metal 6s states is observed at the bottom of the valence band. The measurements are consistent with the results of density functional theory calculations.