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.