Bonding Analysis of Diazoalkenes and Alkylidene Ketenes: Carbone Analogues or Not?

We present a comprehensive bonding analysis of diazoalkenes and their isoelectronic counterparts, alkylidene ketenes. Although both feature a formal C-C-X framework (X = N₂ or CO), reminiscent of classical L→C(0)←L carbones, their bonding characteristics differ significantly. In both systems, the central carbon engages in two electron-sharing interactions: one with the NHC ligand and the other with the terminal heteroallene moiety (N₂ or CO). Bonding metrics, including Wiberg bond indices and Laplacian electron density analyses, further support this distinction, revealing significant electron sharing and continuous charge delocalization across the triatomic C-C-X frameworks. These findings challenge the classical double-dative description of these systems. Additionally, EDA-NOCV analysis confirms that orbital interactions dominate the total attractive energy with σ- and π-type electron-sharing interactions. To rationalize their unexpected stability, we propose a dual π-delocalization model: (i) in-plane π-conjugation across the C-C-X unit and (ii) out-of-plane π-backdonation from the central carbon to the terminal heteroallene. The cooperative delocalization mechanism collectively accounts for the unique bonding patterns and enhanced thermodynamic stability of these otherwise highly reactive main-group species.