Chromium complexes, cations, with

Recently, Lacour, Sauvage and coworkers were able to show that the association of chiral [CuL2] complexes (L=2-R-phen,6-R-bpy and2-iminopyridine) with TRISPHAT 8 leads to an NMR enantiodifferentiation, which allows the determination of the kinetics of racemization of the complexes (bpy=2,2 -bipyri-dine phen=l,10-phenanthroline) [119]. This type of application has recently been reported in conjunction with chiral sandwich-shaped trinuclear silver(l) complexes [122]. Several reports, independent from Lacour s group,have confirmed the efficiency of these chiral shift agents [123-127]. Finally, TRISPHAT can be used to determine the enantiomeric purity of (r] -arene)chromium complexes. These results broaden the field of application of 8 to chiral neutral, and not just cationic, species [114,128,129]. 

Dichromate anions are readily absorbed under acidic conditions by wool that has been dyed with chrome dyes. The chromium(VI) on the fibre is then gradually reduced by the cystine residues in wool keratin to chromium(III) cations, which react with the dye ligands to form a stable complex. In this way the cystine disulphide bonds are destroyed, resulting in oxidative degradation of the wool fibres [71]. 

Substitution reactions also proceed well with cationic t/ -cycloheptadienyliron complexes such as 105 [81] and related chromium complexes [82], and have found applications in natural product synthesis (Scheme 2.38). 

It is also worthy to note that the pentacarbonyl-tungsten complex of the phosphirenylium cation (5) has been recently reported, but no X-ray structure has been obtained. B3LYP/6-311G(d) optimization of the presumed -Cr(CO)5 complex of 5 resulted in an 7 -form (Figure 1) when starting from the bound chromium complex. Such a structure is in complete accord with a delocalized electronic structure of the cation. 

Chromium in the trivalent state forms a variety of salts, the most important and the simplest being the violet salts, which liberate in aqueous solution chromium cation Cr" A green series of chromic salts, isomeric with the violet salts, liberate in aqueous solution some chromium cation, whilst part of the chromium is present as a complex ion. With weak acids, sulphurous, hydrocyanic, or thiocyanic acids, the chromic ion forms complex ions of great stability. Finally, a very large group of salts exists where chromium associated with ammonia forms the complex ion, the chromi-ammines. 

Chromium(II) forms the trigonal bipyramidal complex [CrBr(Me6tren)]Br with the tripod ligand tris(2-dimethylaminoethyl)amine (Me6tren) (Section 35.3.4.3), and pyrazolyl-substituted ligands also form five-coordinate complex cations (Section 35.3.3.4.v see also Table 42). 

All the examples so far have been homonuclear. Complexes (14) provide examples of hetero-nuclear di- -OH cations, with M = Cu, Mn, Co, Ni or Zn, in solution.73 NiI+/Al3+ dimers with di- -OH have been characterized in the solid state, and may well also exist in solution.74 Although cocrystallization of cobalt and chromium hydroxo complexes normally gives the statistically expected mixture of Co2, Cr2 and CoCr di-/i-OH species (difficult to separate), if optically active starting materials are used a very high yield of the mixed dinuclear species [(en)2Cr(jt-OH)2Co(en)2]4+ can be obtained. 

Direct Dyes. These water-soluble anionic dyes, when dyed from aqueous solution in the presence of electrolytes, are substantive to, i.e., have high affinity for, cellu-losic fibers. Their principal use is the dyeing of cotton and regenerated cellulose, paper, leather, and, to a lesser extent, nylon. Most of the dyes in this class are polyazo compounds, along with some stilbenes, phthalocyanines, and oxazines. Aftertreatments, frequently applied to the dyed material to improve washfastness properties, include chelation with salts of metals (usually copper or chromium), and treatment with formaldehyde or a cationic dye-complexing resin. 

To complete this treatment of the manifold types of reactions of the paramagnetic chromium complexes Cr(CO)jI and Cr2(CO)10I (cf. Section II1,D), it remains to discuss their behavior toward liquid NH3 (80). With CrfCOlgl, substitution of three CO ligands by NH3and addition of another NH3 molecule gives /rans-[Cr(CO)2(NH3)JI, which constitutes the first preparation of a cationic CO complex of chromium ... 

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