  Attempts to explain correlated-electron superconductivity have largely
focused on the proximity of the superconducting state to antiferromagnetism.
Yet, there exist many correlated-electron systems that exhibit
insulator-superconducting transitions where the insulating state exhibits
spatial broken symmetry different from antiferromagnetism. Here we focus on a
subset of such compounds which are seemingly very different in which specific
chemical stoichiometries play a distinct role, and small deviations from
stoichiometry can destroy superconductivity. These superconducting materials
share a unique carrier concentration, at which we show there is a stronger than
usual tendency to form local spin-singlets. We posit that superconductivity is
a consequence of these pseudomolecules becoming mobile as was suggested by
Schafroth a few years prior to the advent of the BCS theory.
