Spectroscopic Implications for [N-X-N]⁺ (X = I, Br)-Type Halonium Compounds with Formal Hypervalency

Bis(pyridine)halonium complexes, [Py₂X]⁺ (Py = C₅H₅N, X = I, Br), are formally regarded as hypervalent species, possessing 10 valence electrons at the halogen center in Lewis dot structures. In this study, gas-phase UV spectra of [Py₂X]⁺ complexes were obtained using a cryogenic ion trap. Distinct vibronic structures attributed to charge-transfer (CT) transitions with electron density from the X atom to the terminal Py rings were clearly observed, indicative of halogen bonds. In contrast, transitions between "three-center, four-electron (3c-4e) bond" orbitals, composed of the p orbitals of the X and two N atoms, were observed as broad spectral bands at higher energy regions. Based on these contrasts, we propose that the electronic states of [Py₂X]⁺ can be described by substantial charge-resonance (CR) interactions between two charge-localized states, [(PyX)⁺-Py] and [Py-(XPy)⁺]. Moreover, the well-resolved vibronic bands for the CT transitions enabled the quantitative estimation of the effective N-X bond force constants (at the excited CT state), 159 and 132 N·m^-1 for X = Br and I, respectively, where the cause of this difference is associated with the magnitude of electron transfer upon the CT excitation. This study provides a new perspective on hypervalency from the spectroscopic viewpoint.