Back-Bonding in Ruthenium Porphyrins As Monitored by Resonance Raman Spectroscopy

Resonance Raman spectra are reported for Ru¹¹complexes of octaethylporphyrin (OEP) and tetraphenylporphine (TPP) with pyridine, methanol, and CO axial ligands, using both B- and Q-band excitation. For the porphyrin skeletal mode frequencies above 1370 cm^-1 agree remarkably well with the values calculated on the basis of the porphyrin core size by using parameters derived earlier for iron protoporphyrin complexes. These frequencies shift both positively and negatively when the CO is replaced by pyridine due to from Ru to the porphyrin orbitals. The shift pattern is the same as that observed for the adduct of iron(II) protoporphyrin, relative to the core size predictions, and the extent of the shifts is very similar in the two cases. Thus, to the porphyrin appears to be quantitatively similar for Ru¹¹and shifts are also reported for RuⁿTPP(py)₂. For the stretching, bending, and C-O stretching modes are observed at 513, 578, and 1930 cm^-1. For Fe¹¹porphyrins, the M-CO and C-0 frequencies are somewhat lower and higher, respectively, implying greater back-donation to the bound CO for Ru¹¹than for Fe¹¹.