Chromium complexes, absorption 0-0 band

Electronic Circular Dichroism In contrast to most organic compounds in which CD measurements are limited to the ultraviolet region, most metal complexes possess d-d absorption bands in the more accessible visible and near-infrared regions, allowing for relatively easier application of electronic circular dichroism (ECD) measurements. In fact, the first observation by Cotton of optical rotation measurements through an absorption band and interpretation in terms of differential absorption of the circularly polarized beam was performed on solutions of L-tartrate chromium(III) and copper(II) complexes.100... 

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...

Ti, P) furthermore, none of these states cross each other as the strength of the interaction changes. As an example, we take the case of [Cr(H,0),] +. The aqueous solutions of salts of trivalent chromium are green in colour as a result of absorption bands at 17 000, 24 000, and 37 000 cm (there are also two very weak spin forbidden bands at 16 000 and 22 000 cm ). If the complex is specified by fitting the transitions TtjF) At,(F) and to 17 000 and... 

Low-spin Cr(II) complexes are octahedral with tetragonal distortion (distorted from Oi, to Z>4/, symmetry). They show two absorption bands, one in the visible and one in the near-infrared region, caused by this distortion. In a pure octahedral field, there should be only one d-d transition (see Chapter 11 for more details). Cr(II) also forms dimeric complexes with Cr — Cr bonds in many complexes. The acetate, Cr2(OAc)4, is an example in which the acetate ions bridge between the two chromiums, with significant Cr—Cr bonding resulting in a nearly diamagnetic complex. 

The chromium(O) complex [Cr(N2)2(PMe3)4] is not well established because it decomposes at room temperature with liberation of PMes and N2. Since there are two IR absorption bands at 1990 and 1918 cm" [v(N N)] it has been assigned a cis octahedral structure. The dark brown complex can be prepared from anhydrous CrCb by Scheme 7 or directly from the reactants. There is no spectroscopic or structural information on the intermediates [Cr Cl3(PMe3)3] and [Cr Cl2(PMe3)3]. "... 

From Figure 2, one spin-allowed d-d transition is expected for a regular octahedral complex, and three such transitions for a tetragonally distorted complex. Experimentally, most high-spin chromium(II) complexes exhibit one absorption band (V2, the main band) with a weaker band or shoulder (vi, the distortion band) to lower wavenumber Vi is usually assigned to the transition and V2 to superimposed transitions. There are also... 

The charge transfer absorption band of the chromium to benzene ring transition in 28 and its derivatives is found at considerably longer wavelengths than in the reference complex 158a (Fig. 38) [29, 40, 123]. The corresponding bis(tricarbonylchromium) complexes show a further bathochromic shift [29,40,... 

In aqueous solution, these effects have been clearly observed in the case of the chromium (III) and indium systems. As the indium system has been the object of Raman investigations (2, 3) with conflicting results, it is of special interest that we find three new peaks at 1136 K, 1050 K and 975 K, i.e. corresponding to a sulphato complex of C3V symmetry. The 1050 K absorption cannot be caused by HSO as this ion has an absorption band at 1200 K too, which was not observed. For solutions of other systems of such a composition that the first complex should dominate only a broadening of the z/ -band was observed, together with a very feeble v absorption. These systems included the sodium and the cerium ones. 

Chromium trimethoxide was obtained by the photooxidation of aryl tricarbonyl chromium in methanol the valency of chromium in the product was established by absorption spectra and magnetic studies. Hagihara and Yamazaki synthesized chromium tetra-tert-butoxide by heating a mixture of te(benzene)chromium and di-fert-butylperoxide in the presence of benzene or petroleum ether in a sealed tube at 90° for about 20 h. The excess of solvent was removed and the green residue was sublimed in vacuo. It was found that this residue was contaminated with a small amount of biphenyl as an impurity which was removed as a complex by the addition of 2,4,7-trinitrofluorenone. The blue product thus obtained showed no characteristic absorption bands due to biphenyl and was characterized as chromium tetra-terr-butoxide Cr(OBu )4. 

The second band, which is pink and resembles permanganate ion in color, can be readily washed from the column with about 0.5 M sodium perchlorate (pH = 2-3). This species exhibits two broad, weak absorption bands of nearly equal intensities located at 393 and 525 m//, which are identical to the bands attributed to Cr(CN)(H20)5 by Espenson, who prepared this complex by the reduction of a monocyanocobalt(III) complex with chromium(II) in an acidic aqueous solution. This pink complex has a CN /Cr ratio of 1.0. Finally, the dark band at the top of the column can be eluted with a concentrated, acidified solution of sodium perchlorate. This species has a three-band spectrum identical to that of Cr(H20)6 ... 

This complex was found to constitute 20 % of the hydrolysis mixture and can be eluted from the column with 0.6 M sodium perchlorate solution adjusted to pH 2.0. A chromium to cyanide ratio of 1 1.00 was determined by analysis for this pink, permanganate-colored species. The visible and ultraviolet spectrum of cyanopentaaquochromium(III) ion is independent of pH in the regions from 2 to 3.5 and is included in Fig. 4. The positions of the maxima (in mp) and corresponding molar absorbancy indices (in / mole cm ) in 0.6M sodium perchlorate are 525 (26.0), and 393 (20.5). The strong absorption band in the ultraviolet shows a point of inflection a = 268 I mole" cm" ) at 235 mp. The positions of the absorption maxima are identical to those reported by Espenson. Dilute solutions (pH 2) of the monocyano complex can be stored at 0° for several days without serious decomposition. 

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