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

Syntheses of the copper and gold complexes [Cu{Fe(CO) } ][SbF ] and [Au{Fe(CO) } ][HOB{3,5-(CF ) C H } ] containing the homoleptic carbonyl cations [M{Fe(CO) } ] (M=Cu, Au) are reported. Structural data of the rare, trimetallic Cu Fe, Ag Fe and Au Fe complexes [Cu{Fe(CO) } ][SbF ], [Ag{Fe(CO) } ][SbF ] and [Au{Fe(CO) } ][HOB{3,5-(CF ) C H } ] are also given. The silver and gold cations [M{Fe(CO) } ] (M=Ag, Au) possess a nearly linear Fe-M-Fe' moiety but the Fe-Cu-Fe' in [Cu{Fe(CO) } ][SbF ] exhibits a significant bending angle of 147° due to the strong interaction with the [SbF ] anion. The Fe(CO) ligands adopt a distorted square-pyramidal geometry in the cations [M{Fe(CO) } ] , with the basal CO groups inclined towards M. The geometry optimization with DFT methods of the cations [M{Fe(CO) } ] (M=Cu, Ag, Au) gives equilibrium structures with linear Fe-M-Fe' fragments and D symmetry for the copper and silver cations and D symmetry for the gold cation. There is nearly free rotation of the Fe(CO) ligands around the Fe-M-Fe' axis. The calculated bond dissociation energies for the loss of both Fe(CO) ligands from the cations [M{Fe(CO) } ] show the order M=Au (D =137.2 kcal mol )>Cu (D =109.0 kcal mol )>Ag (D =92.4 kcal mol ). 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) ]→M ←[Fe(CO) ] donation is significantly stronger than the [Fe(CO) ]←M →[Fe(CO) ] backdonation. Inspection of the pairwise orbital interactions identifies four contributions for the charge donation of the Fe(CO) 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.