Possible anionic clusters and mixed valence effects in transition metal chalcogenides and oxides

The formation of anionic clusters similar to O₂^2?, S₂^2?, Se₂^2? but with higher internuclear distances can be responsible for the more specific forms of interaction required to explain the properties of UO₂₊x, and many transition metal chalcogenides such as Fe_1?xS, Ni_1?x,S etc. Resonant structures can lead to clusters of this type even in stoichiometric chalcogenides with NiAs-type structure due to the $\text{d}\text{p}$ mixing, and low energy differences between metal d- and chalcogen p-bands. The mixed valence effects involved are discussed qualitatively.