  We put forward the hypothesis that cations with 6s electrons (Hg,Tl,Pb,Bi) in
the charge reservoir layers of high Tc cuprate superconductors actively
participate in the pairing interaction as negative-U centers. We further argue
that the Hg-cuprates are outstanding superconductors (Tc > 160 K) because they
can exist as two-site negative-U centers, Hg2+2. Their electrons are less
localized than in single-site centers (negative-U or bipolaron) and can have a
strong pairing interaction with a smaller increase in effective mass. The Hg2+2
centers are oriented in the x and y directions and can have phase differences
compatible with the d-wave symmetry of the CuO2 planes.
