Resonance Raman spectroscopy of transition metal protoporphyrin IX dimethyl ester complexes and its interpretation in terms of d-orbital interaction
Abstract
Resonance Raman (RR) spectra are reported for CoII, NiII, CuII, ZnII, PdII, and CdII protoporphyrin IX dimethyl ester (PP) complexes. The reduction in frequency of their structure-sensitive bands are in the order NiII > CoII > PdII > CuII > ZnII > CdII, while the values of λmax for their electronic absorption spectra increase in the order PdII < NiII < CoII < CuII < ZnII < CdII. Empirical assignments for some of the stronger Raman bands of some metalloporphyrins are given. A detailed examination of the interaction between metal ions and the porphyrin ring shows that in planar metalloporphyrins, in addition to the fundamental co-ordinate bonds, both core-expansion effects and π-back-bonding effects exist. When the outer dx2–y2 orbital is occupied by metal electrons, core expansion predominates; if no outer dx2–y2 orbital electron is present, π-back-bonding contributes more than core expansion. Using this assumption, we can explain the spectral changes in both Raman and electronic absorption spectra satisfactorily. We suggest that the essence of the core-expansion mechanism is the multiplex result of the interactions between the outer electrons of metal ions with the electrons of the porphyrin ring and their steric interactions.