Chromium complexes, absorption states

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."... 

Both absorption and emission spectra have been recorded for a variety of octahedral chromium(III) complexes. For the systems of interest here, A/B 2. Inspection of Figure 2 leads to the expectation of three spin-allowed, parity-forbidden transitions between the iA2g and the other quartet states and two spin- and parity-forbidden transitions between the iA2g and the 2Eg and 2T2g states. Aqueous solutions of Cr(H20)s3+ display three bands with e 15 at 17,400, 24,500, and 38,000 cm-1, assigned respectively to the transitions iA2g- iT2g,... 

In summary, spectroscopic studies show that at low temperatures higher excited states of chromium(III) complexes undergo rapid internal conversion to the metastable T2g or 2Eg levels. Intersystem crossing from the 4T2g to the aEg level occurs with near unit efficiency in many chromium(III) complexes. Phosphorescence competes, usually unfavorably, with radiationless decay from the 2Eg state. Studies of the excited states of Mo(III), (4d)3, based on absorption spectra of its complexes in solution, have recently been reported.134-137... 

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. 

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... 

Transient absorption measurements have recently been recorded from the organometallic species chromium hexacarbonyl in ethanol solution [94], Absorption of a 65-fs, 310-nm excitation pulse was followed by measurement of excited-state absorption of a 65-fs, 480-nm probe pulse. The data shown in Figure 14 indicate a rapid nonexponential decay at short times followed by a gradual exponential rise. The slower feature was observed previously [95] and is known to correspond to the solvent complexation of Cr(CO)5 to yield Cr(CO)j(MeOH). The initial feature, which is observed at other probe wavelengths as well, is believed to correspond to the initial ligand loss reaction. Note that this case is different from ICN in that the initially excited wavepacket is not on the side of the Sj potential but rather (as is clear from the molecular symmetry) on a local potential maximum. The wavepacket must then spread that is, dissociation along either direction is equally likely. The rapid nonexponential decay was analyzed in terms of classical kinematics along a dissociative potential. 

With respect to Cr a distinction should be made between Cr(III), which is the common oxidation state in the soils, being rather immobile and so toxic, and Cr( VI), which is very mobile and very toxic. With respect to Hg, the situation is even more complex, due to the occurrence of mercuric mercury (Hg- ), mercurous mercury (Hg2 +), elemental mercury (Hg ) and organic mercury species, such as methyl mercury, (CH3)2Hg (see Chapter 8, Section 2). Furthermore, volatilization of elemental mercury and organic mercury species is common. A description of these processes, in combination with other interactions of Hg in soil, such as reduction, absorption and complexation, is extremely difficult and the approach can only be considered as very approximate for mercury. This also holds to a lesser extent for chromium. 

This work lead to an interest in the luminescence of lower symmetry chromium ammine complexes. In these complexes, the 2Eg state is split into two components by the lower symmetry ligand field. The tetragonal ligand field parameters for many of these compounds were well known or easily available from optical spectroscopy, but the splitting of the 2Eg as measured by the absorption and emission spectroscopy... 

Chromiumfiiij.—Papers concerned with the kinetics and mechanisms of chromium(in) complexes are reviewed in the Inorganic Reaction Mechanisms Specialist Periodical Report Reviews of the absorption and emission spectra chromium(iii) complexes are reviewed in the Inorganic Reaction Mechanisms published. A model based on the antibonding properties of excited electronic states has been developed which correctly predicts the types of photoreaction and relative quantum yields of chromium(iii) and other transition-metal complexes. ... 

This is followed by the transition of chromium from Cr(VI) to a low state of oxidation, and an interaction process with the hydrophilic polymer to form a chromium/polymer complex in which binding between the polymer and photoreleased chromium compound involves either primary forces or physical forces of absorption. The resulting complex causes a solubility decrease in the aqueous system used for development. 

The nature of the anion making up the Cr(III) salt, as well as the gastrointestinal tract contents and the nutritional status of the animal, are important determinants of efficiency of absorption from the gastrointestinal tract. For example, oxalates are better absorbed than phytates (Chen et al. 1973). There is a suggestion that biologically active complexes of Cr(III) are even better absorbed, perhaps to the extent of 10%-25% (Mertz and Roginski 1971). Zinc status also influences chromium absorption (Hahn and Evans 1975). Both Cr(III) and Cr(VI) salts are better absorbed in the fasting than in the fed state (Mackenzie et al. 1959). 

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