Chemical Bonding in Homoleptic Carbonyl Cations [M{Fe(CO)₅ }₂ ]⁺ (M=Cu, Ag, Au)

Syntheses of the copper and gold complexes [Cu{Fe(CO)₅ }₂ ][SbF₆ ] and [Au{Fe(CO)₅ }₂ ][HOB{3,5-(CF₃ )₂ C₆ H₃ }₃ ] containing the homoleptic carbonyl cations [M{Fe(CO)₅ }₂ ]⁺ (M=Cu, Au) are reported. Structural data of the rare, trimetallic Cu₂ Fe, Ag₂ Fe and Au₂ Fe complexes [Cu{Fe(CO)₅ }₂ ][SbF₆ ], [Ag{Fe(CO)₅ }₂ ][SbF₆ ] and [Au{Fe(CO)₅ }₂ ][HOB{3,5-(CF₃ )₂ C₆ H₃ }₃ ] are also given. The silver and gold cations [M{Fe(CO)₅ }₂ ]⁺ (M=Ag, Au) possess a nearly linear Fe-M-Fe' moiety but the Fe-Cu-Fe' in [Cu{Fe(CO)₅ }₂ ][SbF₆ ] exhibits a significant bending angle of 147° due to the strong interaction with the [SbF₆ ]^- anion. The Fe(CO)₅ ligands adopt a distorted square-pyramidal geometry in the cations [M{Fe(CO)₅ }₂ ]⁺ , with the basal CO groups inclined towards M. The geometry optimization with DFT methods of the cations [M{Fe(CO)₅ }₂ ]⁺ (M=Cu, Ag, Au) gives equilibrium structures with linear Fe-M-Fe' fragments and D₂ symmetry for the copper and silver cations and D_4d symmetry for the gold cation. There is nearly free rotation of the Fe(CO)₅ ligands around the Fe-M-Fe' axis. The calculated bond dissociation energies for the loss of both Fe(CO)₅ ligands from the cations [M{Fe(CO)₅ }₂ ]⁺ show the order M=Au (D_e =137.2 kcal mol^-1 )>Cu (D_e =109.0 kcal mol^-1 )>Ag (D_e =92.4 kcal mol^-1 ). The QTAIM analysis shows bond paths and bond critical points for the M-Fe linkage but not between M and the CO ligands. The EDA-NOCV calculations suggest that the [Fe(CO)₅ ]→M⁺ ←[Fe(CO)₅ ] donation is significantly stronger than the [Fe(CO)₅ ]←M⁺ →[Fe(CO)₅ ] backdonation. Inspection of the pairwise orbital interactions identifies four contributions for the charge donation of the Fe(CO)₅ ligands into the vacant (n)s and (n)p AOs of M⁺ and five components for the backdonation from the occupied (n-1)d AOs of M⁺ into vacant ligand orbitals.