Paramagnetic cobalt complexes

There are a few examples of spin equilibria with other metal ions which have not been mentioned above. In cobalt(III) chemistry there exist some paramagnetic planar complexes in equilibrium with the usual diamagnetic octahedral species (22). The equilibria are the converse of the diamagnetic-planar to paramagnetic-octahedral equilibria which occur with nickel(II). Their interconversions are also presumably adiabatic. Preliminary observations indicate relaxation times of tens of microseconds, consistent with slower ligand substitution on a metal ion in the higher (III) oxidation state (120). 

Apart from the platinum and nickel compounds with cr-bonded alkyl or aryl ligands, Chatt s group reported between 1961 and 1966 the synthesis of an impressive series of related cobalt(II), iron(II), rhodium(III), iridium(III), rhe-nium(III) and rhenium(V) complexes [39-42]. The paramagnetic cobalt(II) and iron(II) derivatives MR2(PR 3)2 were stable when R was an aryl group with... 

There are also reports on substantial deviations from proportionality between chemical shifts and the inverse of the longest wavelength. Yamasaki et al. (225) have found that sulphur-coordinated cobalt complexes do not obey this linear relation. Tris(ethylxanthato)-cobalt(iii) as well as other CoS -type complexes are far more shielded than one would expect on the basis of the UV absorption maxima, clearly showing the importance of the radial parameter in the paramagnetic shielding term (which has been considered in the Lucken... 

The conclusion that the cobalt and iron complexes 2.182 and 2.183 are formally TT-radical species is supported by a wealth of spectroscopic evidence. For instance, the H NMR spectrum of the cobalt complex 2.182 indicated the presence of a paramagnetic system with resonances that are consistent with the proposed cobalt(III) formulation (as opposed to a low-spin, paramagnetic cobalt(IV) corrole). Further, the UV-vis absorption spectrum recorded for complex 2.182 was found to be remarkably similar to those of porphyrin 7r-radicals. In the case of the iron complex 2.183, Mdssbauer spectroscopy was used to confirm the assignment of the complex as having a formally tetravalent metal and a vr-radical carbon skeleton. Here, measurements at 120 K revealed that the formal removal of one electron from the neutral species 2.177 had very little effect on the Mdssbauer spectrum. This was interpreted as an indication that oxidation had occurred at the corrole ligand, and not at the metal center. Had metal oxidation occurred, more dramatic differences in the Mdssbauer spectrum would have been observed. 

The intramolecular hydroacylation of 1-pentenal is catalyzed by cobalt complexes such as [Co(PPh3)2(dppe)] or [Co2(/i.-N2)(PPh3)6] and leads to the formation of cyclopentanone in 100% selectivity and 95% conversion. A possible intermediate, the paramagnetic 17-electron pentanal complex (46), has been isolated. A mechanism has been suggested in which the oxidation state of Co alternates between 0 and II, in contrast to earlier mechanistic schemes which assume Co(0),... 

Electron spin resonance spectra provide direct information about paramagnetic metal complexes. Much experimental work has been done on cobalt(II) Schiff-base complexes and their 0 - adducts. Some related... 

ISOTROPIC NMR SHIFTS IN PARAMAGNETIC COBALT AND NICKEL PHOSPHINE COMPLEXES. 

A very broad EPR line (Fig.4) for the SAP matrix with incorporated complexes centered at 1500 G is due to paramagnetic centers with spin value S=3. A similar signal is observed for high-spin tetrahedral Schiff base cobalt complexes [17]. Besides, two additional superimposed EPR lines are observed in tUs spectrum. 

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