Gallium halides as alternative ligands to CO and N₂ in transition metal complexes: A bonding analysis

Quantum chemical calculations using gradient-corrected DFT at the BP86/TZ2P+ level have been carried out for the gallium halide complexes [Fe(CO)₄(GaX)] (X = F-I) and for [Fe(CO)₅] and [Fe(CO)]₄(N₂)]. The nature of the metal-ligand bond has been investigated with an energy decomposition analysis. The Fe-GaX bonds in [Fe(CO)₄(GaX)] have rather high bond dissociation energies that are smaller than the BDE of [Fe(CO)₅] but larger than the BDE of [Fe(CO)]₄(N₂)]. The axial isomer of [Fe(CO) ₄(GaF)] is predicted to be slightly more stable than the equatorial form, while the equatorial isomers of the heavier halogen systems [Fe(CO) ₄(GaX)] (X = Cl-I) are a bit lower in energy than the axial form. The energy difference between the two isomers is for all gallium complexes [Fe(CO)₄(GaX)] smaller than 1.0 kcal/mol. The ratio of covalent bonding and electrostatic attraction in the (CO)₄Fe-GaX bonds is very similar to the values that are calculated for the (CO)₄Fe-CO and (CO)₄Fe-N₂ bonds. The analysis of the bonding situation reveals that the σ-donation of the ligands GaX is much stronger than the π-back-donation, while the (CO)₄Fe-CO and (CO)₄Fe- N₂ bonds possess equally strong contributions from σ-donation and π-back-donation. The GaX and N₂ ligands induce stronger Fe-CO_trans bonds in the axial isomers of [Fe(CO)₄(GaX)] and [Fe(CO)]₄(N₂)]: L(CO)₃Fe→CO _trans π-back-donation.