σ-dative and π-backdative phenyl cation-dinitrogen interactions and opposing sign reaction constants in dual substituent parameter relations

For the overwhelming number of reactions studied with dual substituent parameter treatments, the ratio of the reaction constants rho(R)/rho(F) = lambda is positive and close to unity. Dediazoniations are prominent representatives of the very few unusual reactions for which dual substituent parameter (DSP) relations yield reaction constants of opposing sign. To understand this exceptional behavior, we have studied with ab initio methods the energetic, structural, and electronic relaxations along the unimolecular, linear dediazoniation path of benzenediazonium ions X-l to form phenyl cation X-2 in detail for the parent system and two important derivatives (X = H, NH2, NO2). The results support the electron density based model that describes CN bonding in X-l by synergistic sigma-dative N --> C and C --> N pi-backdative bonding. The analysis provides a theoretical basis for the interpretation of the opposing sign DSP relationship and, in addition, furnishes details about the electronic structure that cannot be deduced from physical-organic studies alone. Polarizations in the sigma-frames critically affect structures (Q values) and electronic structures (populations), and consistent explanations of structural and energetic relaxations in the course of the dediazoniation reactions require their explicit consideration. The classical tool of,pi-electron pushing does not suffice to provide a correct account of the electronic structures. In particular, the analysis resolves the apparent paradox that the amino group can function as an electron donor even though it is negatively charged. If sigma-polarizations dominate in cases where they counteract pi-effects, it would seem reasonable to assume that they also are of comparable magnitude where sigma- and pi-effects act in concert. In the later case, explanations based on;pi-polarizations might therefore seem consistent but they might lack a physical basis.