Superoxides paramagnetism

This could account for the paramagnetism, but esr evidence shows that the 2 cobalt atoms are actually equivalent, and X-ray evidence shows the central Co-O-O-Co group to be planar with an 0-0 distance of l3l pm, which is very close to the 128 pm of the superoxide, 02, ion. A more satisfactory formulation therefore is that of 2 Co atoms joined by a superoxide bridge. Molecular orbital theory predicts that the unpaired electron is situated in a rr orbital extending over all 4 atoms. If this is the case, then the jr orbital is evidently concentrated very largely on the bridging oxygen atoms. 

Similarity with cobalt is also apparent in the affinity of Rh and iH for ammonia and amines. The kinetic inertness of the ammines of Rh has led to the use of several of them in studies of the trans effect (p. 1163) in octahedral complexes, while the ammines of Ir are so stable as to withstand boiling in aqueous alkali. Stable complexes such as [M(C204)3], [M(acac)3] and [M(CN)5] are formed by all three metals. Force constants obtained from the infrared spectra of the hexacyano complexes indicate that the M--C bond strength increases in the order Co < Rh < [r. Like cobalt, rhodium too forms bridged superoxides such as the blue, paramagnetic, fCl(py)4Rh-02-Rh(py)4Cll produced by aerial oxidation of aqueous ethanolic solutions of RhCL and pyridine.In fact it seems likely that many of the species produced by oxidation of aqueous solutions of Rh and presumed to contain the metal in higher oxidation states, are actually superoxides of Rh . ... 

The rhodium(II) compound is a diamagnetic dimer with oxygen it forms a paramagnetic monomeric 02 adduct, probably a superoxide complex represented as (porph)Rh3+02. 

However, reduced ceria is able, alone, to dissociate NO. Martinez-Arias et al. [85] have first investigated by electron paramagnetic resonance (EPR) and FTIR spectroscopies NO reaction on ceria pre-outgassed at different temperatures and showed the role of superoxides differentially coordinated in the formation of hyponitrites species further decomposed into NzO. Later Haneda et al. [86] have demonstrated that reduced ceria and reduced praseodymium oxide dissociate NO even though the presence of a noble metal (Pt) significantly increases the formation of N2 or N20. The main results of this study are summarized in Table 8.9. 

Cardounel AJ, Dumitrescu C, Zweier JL, and Lockwood SF. 2003. Direct superoxide anion scavenging by a disodium disuccinate astaxanthin derivative Relative efficacy of individual stereoisomers versus the statistical mixture of stereoisomers by electron paramagnetic resonance imaging. Biochemical and Biophysical Research Communications 307(3) 704-712. 

Hyperfine interaction has also been used to study adsorption sites on several catalysts. One paramagnetic probe is the same superoxide ion formed from oxygen-16, which has no nuclear magnetic moment. Examination of the spectrum shown in Fig. 5 shows that the adsorbed molecule ion reacts rather strongly with one aluminum atom in a decationated zeolite (S3). The spectrum can be resolved into three sets of six hyperfine lines. Each set of lines represents the hyperfine interaction with WA1 (I = f) along one of the three principal axes. The fairly uniform splitting in the three directions indicates that the impaired electron is mixing with an... 

R. Nilsson, F.M. Pick, and R.C. Bray, Electron paramagnetic resonance studies on reduction of oxygen to superoxide by some biochemical systems. Biochim. Biophys. Acta. 192, 145-148 (1969). 

R.C. Bray, F.M. Pick, and D. Samuel, Oxygen-17 hyperfine splitting in the electron paramagnetic resonance spectrum of enzymically generated superoxide. Eur. J. Biochem. 15, 352-355 (1970). 

---