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We develop a theory describing a parameter based phase diagram to beassociated with materials incorporating skipped valence ions\cite{Varma}. Weuse a recently developed tight-binding approach for the bismuthates to studythe phase diagram exhibiting the crossover from a bond disproportionated (BD)to a charge disproportionated (CD) system in addition to the presence of a newmetallic phase. We argue that three parameters determine the underlying physicsof the BD-CD crossover when electron correlation effects are small: thehybridization between O-2$p_{\sigma}$ and Bi-6$s$ orbitals ($t_{sp\sigma}$),the charge-transfer energy between Bi-6$s$ and O-a$_{1g}$ molecular orbitals($\Delta$), and the width of the oxygen sublattice band ($W$). In the BDsystem, we estimate an effective attractive interaction $U$ between holes onthe same O-a$_{1g}$ molecular orbital. Although here we concentrate on theexample of the bismuthates, the basic ideas can be directly transferred toother perovskites with negative charge-transfer energy, like ReNiO$_{3}$ (Re:rare-earth element), Ca(Sr)FeO$_{3}$, CsTIF$_{3}$ and CsTlCl$_{3}$.

Bond versus charge disproportionation in the bismuth perovskites