Revisiting the Bonding Scenario of Two Donor Ligand Stabilized C₂Species

Quantum chemical calculations using density functional methods were performed for complexes of type L2C2 with L = NHCMe (1), SNHCMe (2) (S = saturated), cAACMe (3), and diamidocarbene (DACMe) (4). The equilibrium structures of 1-4 possess almost linear C4 cores. A high thermochemical stability of the complexes with respect to dissociation, L2C2 → C2 + 2L, is indicated by the large bond dissociation energy following the order 3 > 4 > 2 > 1. The results show that the use of SNHCMe and DACMe as ligands is preferable over NHCMe. The bonding analysis using charge and energy decomposition methods reveals that (cAACMe)2C2 and (DACMe)2C2 possess genuine cumulene C4 moieties, which results from the electron-sharing bonding between quintet L2 and quintet C2 fragments. In contrast, the bonding in (NHCMe)2C2 and (SNHCMe)2C2 comes from a combination of dative and electron-sharing interactions between doublet L2+ and doublet C2- fragments.