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.

Physical, Spectroscopic and Theoretical Results

“…318 The fourteen metal-based electrons are predicted by DFT calculations to occupy the Rh-Rh bonding orbitals in the following order of increasing energy: 2 4 2 * 2 * 4 . The PES (both He(I) and He(II)) supports this, with the * (9.55 eV) and * (9.77 eV) ionizations being so close that their vibrational spreads overlap.…”

Insights Into Metal−π Arene Interactions of the Highly Lewis Acidic Rh24+ Core with a Broad Set of π-Ligands: From Ethylene to Corannulene and C60-Fullerene

“…34 From the photoelectron spectroscopy point of view augmented by density functional calculations (DFT/BLYP), it can only be concluded that the δ* and π* ionization energies are similar within the spread of vibrational energies with excitation (0.2 eV). 35 Herein, we performed calculations by using the DFT level of theory with the new hybrid parameters free exchange-correlation functional PBE0 that is successfully used for modeling transition metal clusters. 36 Our results support a ground-state configuration of σ 2 π 4 δ 2 π* 4 δ* 2 for [Rh 2 (O 2 CCF 3 ) 4 ] (Supporting Information, Figure S33).…”

Tetrakis[μ-N,N′-bis(4-chlorophenyl)formamidinato-N:N′]dimolybdenum(II)

“…Metal±metal interactions are central to the behavior of a wide variety of molecules, clusters, interfaces, and materials. Our interest in the bonding of metal atoms to each other has led us to examine the electronic structures of numerous such systems via gas-phase photoelectron spectroscopy (Lichtenberger et al, 1999(Lichtenberger et al, , 2000. Part of this effort has been focused on understanding the effect of substitution of functional groups at the phenyl rings of the diphenylformamidinate ligand on the electronic structure of metal±metal bonded systems as a whole.…”

“…Data collection: CAD-4 Operations Manual (Enraf±Nonius, 1977); cell re®nement: CAD-4 Operations Manual; data reduction: MolEN (Fair, 1990); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to re®ne structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 1997); software used to prepare material for publication: SHELXTL. Our interest in the bonding of metal atoms to each other has led us to examine the electronic structures of numerous such systems via gas-phase photoelectron spectroscopy (Lichtenberger et al, 1999(Lichtenberger et al, , 2000. Part of this effort has been focused on understanding the effect of substitution of functional groups at the phenyl rings of the diphenylformamidinate ligand on the electronic structure of metal-metal bonded systems as a whole.…”