Diazonium Ions. A Theoretical Study of Pathways to Automerization, Thermodynamic Stabilities, and Topological Electron Density Analysis of the Bonding

Equilibrium geometries and transition-state structures for automerization and thermodynamic stabilities toward loss of N ₂ and N scrambling are reported for the parent alkyl-, alkenyl-, and alkynyldiazonium ions [RN ₂ ] ⁺ (R = methyl ( 1 ), vinyl ( 2 ), and ethynyl ( 3 )). The automerizations of 1 and 2 involve essentially complete disconnection, rotation, and reconnection of N ₂ , but N scrambling in 3 occurs within a bound ion-molecule complex via a two-step process. The degree of unsaturation affects the binding energies in an unexpected fashion; they increase in the order 2 < 1 < 3 . This finding has led to important conclusions regarding the electronic structures of vinyl cations. Topological electron density of the classical vinyl cation shows CC-π-density localization at the CH group and a large positive charge for the CH ₂ group. This result is supported by structural features of β -disubstituted vinyl cations, and it provides a consistent explanation for the lability of 2 , for the higher stability of β -(di)substituted alkenyldiazonium ions, and for the C _β -S _N 2t-type chemistry of the latter. Density integration shows small charges ( < +0.16) for the diazo groups in 1-3 . Electron density accumulation in the CN-bonding region, strong internal polarization of N ₂ , and radial expansion of the density in the NN-bonding region are common features of the electron density distributions of 1-3 . These features are explained with a bonding model invoking synergetic σ -donation from N ₂ to the positively charged hydrocarbon fragment and π -back-donation of comparable magnitude. Implications are discussed regarding the site of nucleophilic attack, the electronic structures of heterosubstituted diazonium ions, and the possibility of stabilizing P ₂ in diphosphonium ions. Of theoretical interest are the occurrences of nonnuclear (3,-3) and noncage (3,+3) critical points in CC triple bonds. The crucial role of the curvature λ3 is emphasized as a parameter for the partitioning of multiple bonds with low polarities.