Metal-metal bonding in Rb-2(O2CCF3)(4): Extensive metal-ligand orbital mixing promoted by filled fluorine orbitals

He I and He II gas-phase photoelectron spectra of Rh-2(O2CCF3)(4) are repor ted. The electron configuration of the metal-metal bond of Rh-2(O2CCF3)(4) is determined to be sigma(2) pi(4) delta(2) delta*(2) pi*(4) with an ioniza tion energy order of sigma approximate to pi > delta > delta* approximate t o pi*. The delta* and pi* ionization energies are similar within the range of vibrational energy separations. Assignment of the Rh-Rh delta ionization is assisted by previous observations that ionizations from delta orbitals in M-2(O2CCF3)(4) (M = Mo. W) and Mo-2(O2CH)(4) show enhanced intensity ove r ionizations from the sigma and pi orbitals with He II excitation. Changes in ionization energies from the dimolybdenum molecule to the dirhodium mol ecule and changes in ionization intensities from He I to He II excitation i ndicate greater metal-ligand mixing in these molecules than observed in oth er dimetal tetracarboxylates. Amsterdam density functional calculations agr ee with the observation that the Rh-Rh delta* and pi* energies are similar. The calculations also indicate substantial ligand mixing into the metal-me tal sigma and pi orbitals. which is enhanced by the trifluoroacetate Ligand despite the inductive withdrawal of electron density by the electronegativ e fluorine atoms. It is found that a specific set of ligand orbitals that p ossess the same symmetries as the Rh-Rh sigma and pi orbitals are destabili zed by overlap interactions with the filled fluorine p orbitals, resulting in greater metal-ligand mixing among these orbitals. The increased mixing e xplains the long-observed enhancement of the delta and delta* ionizations f rom He I to He II excitation for a number of M-2(O2CCF3)(4) (M = Mo. W. Ru) systems.