EDA-NOCV analyses of M−E₂ bond: E₂ binding and activation [E₂ = N₂, H₂]

Dinitrogen and dihydrogen ligated metal complexes [(L)_nM−H₂/N₂] have been known to chemists for nearly four decades. These species are captivating for their unusual bonding interactions between transition metal atoms and closed-shell diatomic molecules like H₂/N₂. Some of these complexes are part of the textbook, with emphasis given to their surprising stability, often without the formation of an electron-sharing M−H₂/N₂ bond. The nature of chemical bonding in these complexes is speculated due to M−H₂/N₂ bond distances and mode of binding (side-on or end-on). In the past, spectroscopic and other tools have studied the nature of the chemical bonds. We report on the energy decomposition analysis coupled with natural orbital for chemical valence (EDA-NOCV) calculations to shed light on the deeper insight of the quantitative pairwise bonding interactions in previously isolated/reported (L)Co−N₂ and (L)Co−H₂ complexes [L = three P- and one E-donor ligand; E = Si, B; Co is either Co(I) or Co(0)]. A comparative EDA-NOCV analysis shows that N₂ is a better π-acceptor while, in contrast, H₂ is a superior σ-donor although both ligands (H₂, N₂) are σ-donor and σ/π-acceptor. The extent of backdonation from Co to H₂/N₂ also depends on E atoms of the chelating ligands (L). The overall intrinsic interaction energy of the Co−N₂ bond is significantly higher by 5–10 kcal/mol than that of the Co−H₂ bond. EDA-NOCV analyses have also studied two Fe−H₂ complexes.