Resonance Raman characterization of five-coordinate dioxygen adducts of porphyrinatocobalt(II) complexes formed in low temperature O-2 matrices

Resonance Raman (RR) spectra of five-coordinate dioxygen adducts of cobalt( tetramesitylporphine) and cobalt(tetramesopropylporphine) formed in low tem perature O-2 matrices are reported. For the first time, all three vibration s expected for the Co-O-2 fragment, nu(O-O), nu(Co-O-2) and nu(Co-O-O), wer e observed for O-16(2) and O-18(2) isotopomers. These bands have been assig ned based on oxygen isotope shift data as well as normal coordinate analysi s of the Co-O-2 moiety. This work presents also the first observation of tw o distinct nu(O-O) bands due to the Co-O-16-O-18 and Co-O-18-O-16 isomers i n 'end-on' type dioxygen adducts. The mechanism of resonance enhancement, t he effect of the axial ligand on the nu(O-O) and nu(Co-O-2) frequencies and the differences in metal-O-2 bonding between cobalt and iron porphyrins ar e discussed.