Several structural features, including electron transfer between atoms of different electronegativity, oxygen deficiency, and unsynchronized resonance of valence bonds, as well as tight binding of atoms and the presence of both hypoelectronic and hyperelectronic elements, cooperatre to confer metallic properties and high-temperature superconductivity on compounds such as (Sr,Ba,Y,La${)}_{2}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{y}}$. .AE