Photoelectron spectroscopy of supported metal-metal interactions.

Abstract

The bonding in a series of ligand-bridged metal dimer complexes has been characterized by He(I) and He(II) photoelectron spectroscopy and approximate molecular orbital calculations. Bridging ligands such as carbonyl, nitrosyl, methylene and pyrazolyl in the complexes [CpFe(NO)]₂, [Cp*Fe(NO)]₂, [CpRu(NO)]₂, [Cp*Co(CO)]₂, [CpFe(CO)₂]₂, [Cp*Fe(CO)₂]₂, [CpFe(CO)]₂-μCO-μCH₂, [Cp*Fe(CO)]₂-μCO-μCH₂, [CpFe(NO)]₂- μCh₂, [CpRu(NO)]₂-μCH₂, [CpCo(CO)]₂-μCH₂, [CpRh(CO)]₂-μCH₂, [Ir(pyrazolyl)(CO)₂]₂, [Ir(3-methylpyrazolyl)(CO)₂]₂ and [Ir(3,5-dimethylpyrazolyl)(CO)₂]₂ are investigated and their effects upon metal-metal interactions are surveyed. Due to the presence of two d⁷ or d⁸ late metal atoms per molecule, these complexes display many overlapping ionization bands in a narrow valence ionization region. Attention has been given to modelling the photoelectron single ionization with asymmetric and symmetric Gaussians. The overlapping ionizations are successfully represented in terms of the model bandshapes. Thermodynamic relationships between bond dissociation and photoelectron ionization energies are also investigated. With relationships of this type, trends in bond energies may be correlated with ionization energies. Ligand inductive and bonding effects as well as small changes in molecular geometry cause shifts in the metal-based ionizations, which aid chemical understanding and interpretation of the molecular orbital picture. By comparing a series of related metal dimers, the assignment of related ionizations in the photoelectron spectra becomes apparent. Changes in ligand π accepting ability and changes in metal and formal oxidation states are also probed. Addition information is provided by vibrational fine structure in Cp₂Os, [CpFe(NO)]₂, and [Cp*Co(CO)]₂ and spin-orbit splitting in Cp₂Os. The metal-ligand backbonding combinations are found to be the most stable interactions and are responsible for the stability of the metal dimers. Metal-metal interactions are found to be relatively unimportant. Ligands with stronger π accepting abilities allow for more stabilized supported metal dimer complexes.