Chromium coordination shell

These maxima in the Fourier transform data, which correspond to the different chromium coordination shells, were isolated using a filter window function. The inverse transform of each peak was generated and fitted using a non-linear least squares program. The amplitude and phase functions were obtained from the theoretical curves reported by Teo and Lee (2 ). The parameters which were refined included a scale factor, the Debye-Waller factor, the interatomic distance, and the threshold energy difference. This process led to refined distances of 1.97(2) and 2.73(2) A which were attributed to Cr-0 and Cr-Cr distances, respectively. Our inability to resolve second nearest neighbor Cr-Cr distances may be a consequence of the limited domain size of the pillars. 

Leonard Katzin I want to make two comments, one on this last point in relation to the point that Dr. Margerum made about substituents. Chromium (I II) in the hexahyd rated state is quite resistant to penetration of the coordination shell by nitrate ion. Yet if one takes the% violet chromium nitrate hexahydrate in solid state and treats it with liquid tributylphosphate, within a matter of minutes one gets chromium compound in solution by the mechanism of substituting tributylphosphate for water. So this reaction is fast. This initial solution is violet Within the space of an hour or two it is green. And we have had for some years now infrared evidence that this color change is accompanied by penetration of the nitrate ion into the coordination sphere (4). So this again is a matter of the substituent s changing the relationship of the water. 

Some recent advances are the isolation of aryl isocyanide complexes of chromium(III) Cr(CNAr)6]X3, and six- and seven-coordinate alkyl isocyanide complexes of chromium(II) Cr(CNR)6j7]X2. All but the chromium(O) and the seven-coordinate chromium(II) complexes lave less than 18 electrons in the valency shell. 

The solvation of chromium(llI) ion in certain mixed-solvent systems has been studied in experiments which are relatively free of ambiguity. The exchange of solvent molecules between the mixed solvent and the solvated species Cr(OH2)w (So)n3+ (So = organic solvent component) is a very slow process. The species with solvation shells having different compositions can be separated from one another by column ion-exchange procedures. Analytical procedures based upon such separations allow evaluation of equilibrium constants for reactions involving replacement of coordinated water by the polar organic component. These equilibrium constants are reviewed in this chapter with attention focused upon the dependence of the equilibrium constants upon solvent composition, and the relationship of relative values of the equilibrium constants to the statistically expected values. 

The inertness of chromium (III) ion has allowed a complete characterization of the solvation of this ion in several mixed-solvent systems. For this inert transition metal ion of charge 3+, concern with the first shell coordination of solvent molecules can be viewed as a subdivision of coordination chemistry, a point of view less easily applicable to labile systems of metal ions with lower charge. Thus the approach used here is different from that used by A. K. Covington and co-workers (16) in their NMR studies of labile systems (e.g., sodium and cesium chlorides in water-methanol solutions (17)). 

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