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 = N 2 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 (N 2 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.