  Metal-oxygen complexes containing Cu,- Tl-, Hg-, Bi- and Pb-cations are
electronically active in superconducting copper-oxides by stabilizing single
phases with enhanced $T_c$, whereas other metal-oxygen complexes deteriorate
copper-oxide superconductivity. Cu, Tl, Hg, Bi, Pb in their actual oxidation
states are closed shell $d^{10}$ or inert $s^2$ pair ions. Their electronic
configurations have a strong tendency to polarize the oxygen environment. The
closed shell $d$ ions with low lying $nd^{10}\leftrightarrow nd^9(n+1)s$
excitations form linear complexes through $d_{z^2}-s$ hybridization polarizing
the apical oxygens. Comparatively low $nd^9(n+1)s$ excitation energies
distinguish $\rm Cu^{1+,3+}, Tl^{3+}, Hg^{2+}$ from other closed shell $d^{10}$
ions deteriorating copper-oxide superconductivity, {\it e.g.} $\rm Zn^{2+}$.
