sigma-dative and pi-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 para meter treatments, the ratio of the reaction constants rho(R)/rho(F) = lambd a is positive and close to unity. Dediazoniations are prominent representat ives of the very few unusual reactions for which dual substituent parameter (DSP) relations yield reaction constants of opposing sign. To understand t his exceptional behavior, we have studied with ab initio methods the energe tic, 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 C N bonding in X-l by synergistic sigma-dative N --> C and C --> N pi-backdat ive bonding. The analysis provides a theoretical basis for the interpretati on of the opposing sign DSP relationship and, in addition, furnishes detail s about the electronic structure that cannot be deduced from physical-organ ic studies alone. Polarizations in the sigma-frames critically affect struc tures (Q values) and electronic structures (populations), and consistent ex planations of structural and energetic relaxations in the course of the ded iazoniation reactions require their explicit consideration. The classical t ool of,pi-electron pushing does not suffice to provide a correct account of the electronic structures. In particular, the analysis resolves the appare nt paradox that the amino group can function as an electron donor even thou gh it is negatively charged. If sigma-polarizations dominate in cases where they counteract pi-effects, it would seem reasonable to assume that they a lso 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.