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We report on the synthesis, the layered triclinic P1 crystal structure, band structure and Fermi surface analysis and preliminary transport and ESR data of β-(EDT-TTF-I2)2(1+x)+[HO2C–CHCH–CO2−](1−x)[−O2C–CHCH–CO2−]x, a metallic radical cation salt of the nominal, mono-deprotonated π-conjugated dicarboxylate anion of fumaric acid with a metal–insulator transition at ca.70 K and conclude that the system is doped, that is, a fraction of the fumarates are further deprotonated to their dianion forms. It is shown that the actual number of protons required within the extended fumarate network to effectively balance the chemical activity at the nuclei/solution interface during crystal growth is compensated by tapping into the HOMO bands serving as a molecular hole reservoir such that the compound formulation and structure remain intact. A dual [proton]/[hole] mixed-valence is thereby revealed and defined as the duality between the modulation of the electrostatic potential of the environment, the incommensurate number and the physics of migrating holes within a molecular system. [ABSTRACT FROM AUTHOR]