The He I valence photoelectron spectra of [Ir(μ-pyrazolyl)(CO)2]2, [Ir(μ-3-methylpyrazolyl)(CO)2]2, and [Ir(μ-3,5-dimethylpyrazolyl)(CO)2]2 have been obtained. These complexes may be viewed as two square-planar d8 iridium centers held together by two bridging pyrazolyl ligands to form a six-membered Ir-(N-N)2-Ir ring. The ring is in a boat conformation with the iridium atoms positioned at the bow and stern such that interaction is possible between the filled dz2 orbitals from each metal center. The 3,5-dimethylpyrazolyl complex is active as a hydrogenation catalyst while the other related complexes are not. It has been proposed previously that greater filled-filled orbital interaction between the metal centers in the dimethyl complex (as caused by the shorter metal-metal distance) accounts for the greater reactivity. It is found here that there is indeed substantial interaction between the dz2 atomic orbitals of the two iridium centers, and the ionization corresponding to the Ir-Ir antibonding dz2-dz2 interaction is the lowest energy ionization band of these complexes. This ionization is cleanly separated from the other ionizations of the complexes. Of particular interest is the broad and unusual asymmetry found in the band profile of the initial ionization, which indicates appreciable vibrational excitation associated with the shortening of the metal-metal distance upon removal of an electron from this molecular orbital. The energy of the first ionization band is very sensitive to the methyl substitutions on the pyrazolyl groups. The sensitivity of this ionization to methyl substitution (and in turn the change in reaction chemistry between these complexes) is due more to the electronic inductive effects of the methyl group substitutions than to changes in geometry and splitting of the bonding and antibonding combinations of the metal dz2 orbitals.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry