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)C6H3}(3)] containing the homoleptic carbonyl cations [M{Fe(CO)(5)}(2)](+) (M=Cu, Au) are reported. Structural data of the rare, trimetallic Cu2Fe, Ag2Fe and Au2Fe complexes [Cu{Fe(CO)(5)}(2)][SbF6], [Ag{Fe(CO)(5)}(2)][SbF6] and [Au{Fe(CO)(5)}(2)][HOB{3,5-(CF3)(2)C6H3}(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 degrees 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 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.

Automated summary

The synthesis of copper and gold complexes containing homoleptic carbonyl cations is reported, along with structural data on rare trimetallic complexes. Silver and gold cations exhibit linear structures, while copper cations show a bending angle due to strong interaction with anions. Fe(CO)(5) ligands adopt different orientations in the cations, with distinct symmetry for each metal cation. Bond dissociation energies and orbital interactions between the ligands and metal cations are also analyzed.