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 (Mq = Ti2–, 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.