Donor-acceptor coordination of anions by chalcogen atoms of 1,2,5-chalcogenadiazoles

Synthetic, structural, and thermodynamic aspects of the recently discovered new reaction, donor-acceptor coordination of anions (A^-) by chalcogen atoms (E) of 1,2,5-chalcogenadiazoles, are considered. According to the quantum chemical calculations, the charge transfer from A^- to the heterocycle via the mechanism of negative hyperconjugation (i.e., from the MO of the lone pair of A^- to the virtual σorbital of the E - N bond of chalcogenadiazole) depends on the nature of E and A^-, being 0.42 - 0.52 and 0.30 - 0.44 e in terms of the Mulliken and NBO methods, respectively. According to the X-ray diffraction data, the E - A^- coordinate bond is always longer than the sum of the covalent radii but shorter than the sum of the van der Waals radii of the atoms forming the bond. The E - A^- bond energy varies in a wide range, from ~25 kcal mol^-1 comparable to the energy of weak covalent bonds (e.g., internal N - N bond in organic azides) to ~86 kcal mol^-1 comparable to the C - C bond energy in organic compounds. The quantum chemical estimations of the thermodynamics of the donor-acceptor coordination of the anions by the chalcogen atoms of 1,2,5-chalcogenadiazoles indicate that for E = Te and Se this reaction may be of general character also covering E = S in some cases.