Back-bonding in ruthenium porphyrins as monitored by resonance Raman spectroscopy

Abstract

Resonance Raman spectra are reported for Ru/sup II/ complexes of octaethylporphyrin (OEP) and tetraphenylporphine (TPP) with pyridine, methanol, and CO axial ligands, using both B- and Q-band excitation. For Ru/sup II/ OEP(CO)(MeOH) the porphyrin skeletal mode frequencies above 1370 cm/sup -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 ..pi..-back-donation from Ru to the porphyrin ..pi.. orbitals. The shift pattern is the same as that observed for the bis(imidazole) 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, ..pi..-back-bonding to the porphyrin appears to be quantitatively similar for Ru/sup II/ and Fe/sup II/. ..pi..-Back-bonding shifts are also reported for Ru/sup II/TPP(py)/sub 2/. For Ru/sup II/OEP(CO)(py) the Ru-CO stretching, Ru-C-O bending, and C-O stretching modes are observed at 513, 578, and 1930 cm/sup -1/. For Fe/sup II/ porphyrins, the M-CO and C-O frequencies are somewhat lower and higher, respectively, implying greater back-donation to the bound CO for Ru/sup II/ than for Fe/sup II/.