We put forward the hypothesis that cations with 6s electrons (Hg,Tl,Pb,Bi) in the charge reservoir layers of high T_c cuprate superconductors actively participate in the pairing interaction as nmegative-U centers. We further argue that the Hg-cuprates are outstanding superconductors (T_c> 160 K) because they can exist as two-ion negative-U centers, Hg_2~+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 Hg_2~+2 centers are oriented in the x and y directions and can have phase differences compatible with the d-wave symmetry of the CuO_2 planes.
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