Cobalt complexes, absorption transitions

Infrared data have been tabulated for benzotriazole and a wide range of its transition metal complexes or adducts (172). Far infrared spectra have been recorded for copper(II) benzotriazole adducts and bands at 270-320 cm-1 have been assigned to Cu-N vibrations (172). Infrared absorptions at approximately 825, 800, and 775 cm-1 in the spectra of cobalt(III)/4,5-disubstituted triazolate complexes have been attributed to triazolate ring vibrations (109). Infrared data have been reported and assignments made for palladium and platinum thiatriazoline-5-thionate complexes (37) and for the parent thione (127). Vibrational spectroscopy has been employed in an attempt to determine coordination sites for a range of 8-azapurine complexes (108). 

Example 10.2-3 Since it is the nearest-neighbor atoms in a complex that determine the local symmetry and the vibronic interactions, irans-d i c h I orob i s( cthylenediamine)cobalt(III) (Figure 10.3(a)) may be regarded as having D4h symmetry for the purpose of an analysis of its absorption spectrum in the visible/near-ultra-violet region (Ballhausen and Moffitt (1956)). The fundamental vibrational transitions therefore involve the 21 — 6 = 15 normal modes of symmetry 2Ate, B,. B2g, Eg, 2A2m, BltJ, 3Em. 

Recent infrared absorption and x-ray diffraction studies have shown that metals of the first transition series normally form M—N bonds with the thiocyanate group.12-14 X-ray diffraction studies have shown that the isothiocyanato groups are trans to each other in the tetrapyridine complexes of iron, cobalt, and nickel 14-17 this configuration has been confirmed for the iron compound by electronic spectra.4... 

Tables 1 and 2 gives the numerical data for a series of vanadium (II), chromium (III), manganese (IV), molybdenum (III), rhenium (IV), iridium (VI), cobalt (II), and nickel (II) complexes. The first spin-allowed absorption band, caused by an internal transition in the partly filled shell, has the wavenumber equal to A. If spin-forbidden transitions are superposed on this band, a certain distortion from the usual shape of Gaussian error curve can be observed, and one takes the centre of gravity of intensity as the corrected wavenumber ai. One has to be careful not to confuse electron transfer or other strong bands with the internal transitions discussed here. Obviously, one has also to watch for absorption due to other coloured species, produced e. g. by oxidation or hydrolysis of the solutions. In the case of certain octahedral nickel (II), and nearly all tetrahedral cobalt (II) complexes, the first band has not actually been...

The electronic absorption spectrum of the complex shows an intense band at 295 nm (e = 18,000), but ligand field (LF) bands were not observed at lower energy. The intense band was assigned as a benzyl-to-cobalt CT transition, and it was proposed that the photoreaction proceeds through population of such a CT state leading to homolytic cleavage of the cobalt-carixtn bond. 

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