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We present a valence transition model for electron- and hole-doped cuprates,within which there occurs a discrete jump in ionicity Cu$^{2+} \to$ Cu$^{1+}$upon doping, at or near optimal doping in the electron-doped compounds and atthe pseudogap phase transition in the hole-doped materials. Doped cuprates havenegative charge-transfer gaps, just as rare earth nickelates and BaBiO$_3$.Because of strong correlations and small $d-p$ electron hoppings the systemsbehave as effective $\frac{1}{2}$-filled Cu-band in the undoped state, and ascorrelated two-dimensional geometrically frustrated nearly $\frac{1}{4}$-filledO-band in the doped state. The theory gives the simplest yet most comprehensiveunderstanding of experiments in the normal states. The robustantiferromagnetism in the conventional T$^\prime$ crystals, the strong role ofoxygen deficiency in driving superconductivity and charge carrier signcorresponding to holes at optimal doping are all manifestations of the samequantum state. In the hole-doped pseudogapped state, a biaxial commensurateperiod 4 charge density wave state of O$^{1-}$-Cu$^{1+}$-O$^{1-}$ spin-singletscoexists with broken rotational C$_4$ symmetry. Finite domains of this brokensymmetry state will exhibit the polar Kerr effect. Superconductivity within themodel results from a destabilization of the $\frac{1}{4}$-filled band pairedWigner crystal [Phys. Rev. B {\bf 93}, 165110 and {\bf 93}, 205111]. We positthat a similar valence transition, Ir$^{4+} \to$ Ir$^{3+}$, occurs inelectron-doped SrIr$_2$O$_4$. We make testable theoretical predictions oncuprates and iridates. Finally, we note that there exist an unusually largenumber of unconventional superconductors that exhibit superconductivityproximate to exotic charge ordered states, whose bandfillings are also$\frac{1}{4}$, exactly where the paired Wigner crystal is most stable.

The Valence Transition Model of Pseudogap, Charge-Order and Superconductivity in Electron- and Hole-Doped Copper Oxides