According to the DFT calculations, [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazole (4), [1,2,5]selenadiazolo[3,4-c] [1,2,5]thiadiazole (5), 3,4-dicyano-1,2,5-thiadiazole (6), and 3,4-dicyano-1,2,5-selenadiazole (7) have nearly the same positive electron affinity (FA). Under the CV conditions they readily produce long-lived pi-delocalized radical anions (pi-RAs) characterized by EPR. Whereas 4 and 5 were chemically reduced into the pi-RAs with thiophenolate (PhS-), 6 did not react and 7 formed a product of hypercoordination at the Se center (9) isolated in the form of the thermally stable salt [K(18-crown-6)] [9] (10). The latter type of reactivity has never been observed previously for any 1,2,5-chalcogenadiazole derivatives. The X-ray structure of salt 10 revealed that the Se S distance in the anion 9(2.722 angstrom) is ca. 0.5 angstrom longer than the sum of the covalent radii of these atoms but ca. 1 angstrom shorter than the sum of their van der Waals radii. According to the QTAIM and NBO analysis, the Se-S bond in 9 can be considered a donor-acceptor bond whose formation leads to transfer of ca. 40% of negative charge from PhS- onto the heterocycle. For various PhS-/1,2,5-chalcogenadiazole reaction systems, thermodynamics and kinetics were theoretically studied to rationalize the interchalcogen hypercoordination vs reduction to pi-RA dichotomy. It is predicted that interaction between PhS- and 3,4-clicyano-1,2,5-telluradiazole (12), whose EA slightly exceeds that of 6 and 7, will lead to hypercoordinate anion (17) with the interchalcogen Te S bond being stronger than the Se-S bond observed in anion 9.