Chromium complexes, absorption spectra

Study of the phosphorescence (and absorption) spectra of chromium(III) complexes has been used to determine to what extent delocalization of metal d electrons onto the ligands is possible. Using the absorption spectrum of Cr(H20)83 +, for example, it is possible to... 

UV irradiation of the homoleptic alkyl complexes of chromium was found to give homolysis of the Cr—alkyl bonds and the generation of free radicals 17, 45, 46). When Cr(nor)4 was irradiated with near-UV light in CCU/hexane mixtures, clean changes in the UV-visible absorption spectrum resulted and norbornane, 1-chloronorbornane, and hexachloroeth-ane were produced [Eq. (56)] (45). An orange, chromium-containing solid... 

T FIGURE 23.30 shows what happens to crystal-field splitting when the ligand is varied in a series of chromium(III) complexes. Because the Cr atom has an [Ar]3d 4s electron configuration, Cr has the configuration [Ar]3d and therefore is a d ion. Consistent with Hund s rule, the three 3d electrons occupy the t2 set of orbitals, with one electron in each orbital and all the spins the same. (Section 6.8) As the crystal field exerted by the six ligands increases, A increases. Because the absorption spectrum is related to this energy separation, these complexes vary in color. 

The peach-colored band containing this species can be eluted from the cation-exchange column with 0.10 M sodium perchlorate solution at pH 2.5. (Analysis of such solutions gave chromium to cyanide ratios of 1 to 2.00 within experimental error.) The visible and ultraviolet absorption spectrum of the dicyanp complex is shown in Fig. 4 and is independent of pH in the region from 2.0 to 4.0. The position of the maxima (in mfi) and corresponding... 

The intermediate cyclooctene complex appears to be more reactive with respect to CS coordination and more sensitive to oxidation when the arene ring bears electron-withdrawing groups (e.g., C02CH3). Dicarbonyl(methyl rj6-benzoate)-thiocarbonyl)chromium is air stable in the solid state and reasonably stable in solution.9 The infrared spectrum exhibits metal carbonyl absorptions at 1980 and 1935 cm"1 and a metal thiocarbonyl stretch at 1215 cm"1 (Nujol) (these occur at 1978, 1932, and 1912 cm"1 in CH2C12 solution).10 Irradiation of the compound in the presence of phosphite or phosphine leads to slow substitution of CO by these ligands, whereas the CS ligand remains inert to substitution. The crystal structure has been published."... 

Under the same conditions, cobalt acetylacetonate afforded a mixture of four products the mono-, di-, and triacetylated chelates (XVII, XVIII, and XIX), along with the starting material. In contrast to the chromium chelates, the mixture of cobalt complexes was cleanly separated by chromatography. The identity of each of these products was established by an NMR spectrum. The presence of uncoordinated carbonyl groups was revealed by infrared absorption at 1675 cm.-1... 

Implicit in the foregoing is the assumption that the oxidation states can be assigned with certainty in the binuclear intermediate, and this issue will now be considered explicitly. In the present case, a strong item of evidence is that the n <- %d absorption, characteristic of the Ru11—heterocycle, is observed in the successor complex an Rum-heterocycle shows no absorption in the same region of the spectrum. The d—d absorption characteristic of Crm in an oxygen environment is also observed. Finally, the rate of aquation at the chromium center is characteristic of the Crm state. 

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