Chameleonic π-Hole Behavior in Metal−Isocyanide Complexes: N ↔ C Composition-Controlled Migration

Abstract

Cocrystal architecture reflects the cumulative outcome of competing and cooperating noncovalent interactions (NCIs) within the lattice. Here we demonstrate that the electrophilic π-hole of a coordinated aryl isocyanide is chameleonic: its localization migrates between the nitrogen and carbon termini of the C≡N bond in direct response to crystal composition. X-ray diffraction analysis of trans-[PdI2(CNC6H4Cl-4)2] (1) and its iodine cocrystals 1·I2 and 1·2I2, complemented by the isostructural platinum(II) analogue 2·I2, reveals three distinct binding regimes: Pd-πAr assembly in the parent complex, Pd-π-(CN) assembly at 1:1 stoichiometry, and π-(CN)-hole···I(LP) contacts at 1:2 stoichiometry. Quantum chemical analysis (Molecular Electrostatic Potential, Quantum Theory of Atoms in Molecules, NCIplot, Energy Decomposition Analysis, Natural Bond Orbital) establishes that the short C≡N bond and the small electrostatic differential between the two termini generate a delocalized π-hole that adapts to spatial demands imposed by competing halogen bonds. Energy decomposition further reveals that the N ↔ C migration is accompanied by a fundamental change in the stabilizing forces, from dispersion- and correlation-dominated stacking to orbital- and electrostatic-driven halogen bonding. These findings establish stoichiometry-controlled π-hole directionality as a design principle for engineering cocrystals in which multiple NCI types coexist in spatially optimized arrangements.