Abstract
Syntheses of the copper and gold complexes [Cu{Fe(CO)(5) }(2) ][SbF(6) ] and [Au{Fe(CO)(5) }(2) ][HOB{3,5-(CF(3) )(2) C(6) H(3) }(3) ] containing the homoleptic carbonyl cations [M{Fe(CO)(5) }(2) ](+) (M=Cu, Au) are reported. Structural data of the rare, trimetallic Cu(2) Fe, Ag(2) Fe and Au(2) Fe complexes [Cu{Fe(CO)(5) }(2) ][SbF(6) ], [Ag{Fe(CO)(5) }(2) ][SbF(6) ] and [Au{Fe(CO)(5) }(2) ][HOB{3,5-(CF(3) )(2) C(6) H(3) }(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) ][SbF(6) ] exhibits a significant bending angle of 147° due to the strong interaction with the [SbF(6) ](-) 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 D(2) 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.