Metal-metal bonding in Rh₂(O₂CCF₃)₄: Extensive metal-ligand orbital mixing promoted by filled fluorine orbitals

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.