We have established a well-defined dome-shaped T-c curve in Ir0.94-xRhxSe2 superconductors. The maximum T-c(onset) of 9.6 K was obtained at x = 0.36, at which the Se-Se separation in the dimer anion is the longest. Simultaneously, the destabilization of Se-Se dimers accompanied by partial electron transfer from the Ir/Rh to the chalcogenide ions resulted in the emergence of optimal T-c.