First principles linear muffin-tin orbital (LMTO) calculations are used to explore the origin of the electronic driving force behind the stabilization associated with metal atom loss in ScS leading to nonstoichiometric Sc1-xS. On oxidation of the remaining Sc atoms, the d-band drops in energy, leading to a stabilization of the occupied sulfur p-band by enhanced metal-sulfur orbital interaction or covalency. Such a mechanism is only applicable for early chalcogenides where the energy of the metal d-band is strongly sensitive to variations in charge.