Metal−Metal Bonding in Rh2(O2CCF3)4: Extensive Metal−Ligand Orbital Mixing Promoted by Filled Fluorine Orbitals

Abstract

He I and He II gas-phase photoelectron spectra of Rh2(O2CCF3)4 are reported. The electron configuration of the metal−metal bond of Rh2(O2CCF3)4 is determined to be σ2 π4 δ2 δ*2 π*4 with an ionization energy order of σ ≈ π > δ > δ* ≈ π*. The δ* and π* ionization energies are similar within the range of vibrational energy separations. Assignment of the Rh−Rh δ ionization is assisted by previous observations that ionizations from δ orbitals in M2(O2CCF3)4 (M = Mo, W) and Mo2(O2CH)4 show enhanced intensity over ionizations from the σ and π orbitals with He II excitation. Changes in ionization energies from the dimolybdenum molecule to the dirhodium molecule and changes in ionization intensities from He I to He II excitation indicate greater metal−ligand mixing in these molecules than observed in other dimetal tetracarboxylates. Amsterdam density functional calculations agree with the observation that the Rh−Rh δ* and π* energies are similar. The calculations also indicate substantial ligand mixing into the metal−metal σ and π orbitals, which is enhanced by the trifluoroacetate ligand despite the inductive withdrawal of electron density by the electronegative fluorine atoms. It is found that a specific set of ligand orbitals that possess the same symmetries as the Rh−Rh σ and π orbitals are destabilized by overlap interactions with the filled fluorine p orbitals, resulting in greater metal−ligand mixing among these orbitals. The increased mixing explains the long-observed enhancement of the δ and δ* ionizations from He I to He II excitation for a number of M2(O2CCF3)4 (M = Mo, W, Ru) systems.