The Nature of Nonclassical Chalcogeno-Carbonyl Ligands

Coordination of chalcogeno-carbonyl ligands CX (X = O, S, Se) to transition metals can induce either expansion (classical) or contraction (nonclassical) of the C–X bond. While nonclassical CO complexes have been extensively studied, the heavier analogs CS and CSe remain comparatively underexplored, despite their higher π-acidity and potential for broader applications. In this work, we used Kohn–Sham molecular orbital theory together with canonical energy decomposition analysis to investigate the nature and bonding characteristics of [M(CO)₅(CX)]^q complexes (M^q = Ti^2–, V^–, Cr⁰, Mn⁺, Fe²⁺). Our results show that nonclassical behavior occurs in the cationic metal complexes, where the trend set by the electrostatic [M]–CX interactions (favoring C–X contraction) dominates the trend set by [M]–CX π-backdonation (favoring C–X expansion), even for the heavier chalcogeno-carbonyl ligands, despite their higher π acidity.