Photoelectron Spectroscopy and Rates of CO Substitution of (η⁵-C₅H₄X)Rh(CO)₂ Compounds. Separation of s and p Ring Substituent Effects

The He I photoelectron spectra for a series of monosubstituted η⁵-C₆H₄X)Rh(CO)₂ compounds are reported (where X = N0₂, CF₂, C1, H, CH₃, NMe₂) and compared to the rates of carbonyl substitution reactions. The carbonyl substitution by phosphine follows an associative mechanism, and the rates are generally inhibited by greater electron richness at the metal center in these compounds. However, the rates for certain substitutions, particularly when X is C1 or NMe₂, are faster than indicated by the inductive characteristics of these groups. The photoelectron spectra of the (η⁵-C₅H4X)Rh(CO)₂ compounds illustrate the effects of X on the electronic structure and rates of substitution. Pronounced shifts are seen in the cyclopentadienyl p and metal d valence ionizations as the X group is varied. The shifts of most of the valence ionizations closely follow the inductive capabilities of the X substituents, as also indicated by correlations with Hammett s values and the carbonyl stretching frequencies of the compounds. Certain ionizations are also affected by orbitale of the X group that have p symmetry with respect to the cyclopentadienyl ring. Thus, the ionization energy shifts provide a relative measure of the inductive and resonance (p) interaction between X and the compound. The rates of CO substitution correlate with the ionizations when the shifts due to both the inductive and resonance (p) effects are taken into account. These results suggest that the “slipped ring” intermediate (η³-C₅H₄X)Rh(CO)₂PPh₃ is stabilized through p derealization on the cyclopentadienyl ring, thus enhancing the rates of substitution.