Unveiling the Bonding Scenario in Metal–Aryne Complexes with EDA-NOCV Analyses

Transition metal-based organometallic compounds have been explained by the Dewar–Chatt–Duncanson (DCD) model, established in 1953, which provides a conceptual framework elucidating the interaction between transition metals and ligands. This interaction involves σ-donation from the ligand to the symmetric vacant d-orbital of the transition metal (TM⃖L), coupled with π-backdonation from a distinctly occupied d-orbital of the transition metal to the suitable empty orbital (mostly antibonding type) on the ligand (TM → L), which leads to the variations in bond lengths in the bonded ligand (typically bond elongation) and vibrational frequencies within ligand bonds (such as C=O, N=N, and C=C of olefins), serving as an indicator of the ligand’s π-accepting strength. One such effective and highly reactive ligand is benzyne/aryne, which is generated in situ and has been stabilized by coordinating to a transition metal. The transition metal–aryne complexes are primarily formed with low-valent early transition metals and late (d¹⁰) transition metals. The findings, on employing the EDA-NOCV calculations of different classical textbook examples of experimentally synthesized mononuclear TM–aryne complexes, specifically TM–benzyne complexes, reveal intriguing deviations from the original DCD model and suggest that the bonding interaction of these well-known organometallic complexes occurs between TM and aryne fragments in their ‘electronically charged doublet states’ (as TM⁺ and aryne^–). Notably, when the TM resides within groups IV–IX of the periodic table, the interaction exhibits one dative σ-bond, one electron-sharing π-bond, and one (a few have two) additional dative σ/π bond (D + E). Even though late TM (d¹⁰, Ni/Pd/Pt) exhibits the potential to form both dative bonds (D) (in accordance with the DCD model) and D + E interaction between electronically charged fragments, it still slightly favors the later bonding scenario. The major contribution in the bond formation of TM–aryne complexes is from electrostatic interaction energy (ΔE_elstat) and the major contribution toward the orbital interaction (ΔE_orb) is dominated by the electron sharing π-bond formation.