Chromium complexes, cations

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. 

SCHEME 84. Addition of functionalized zinc-copper reagents to cationic chromium complexes and subsequent cyclization... 

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). 

Chromium complexes,163 cationic homogeneous complexes of rhodium164 and ruthenium,165 166 and supported palladium complexes141167 have been found to be extremely selective in reduction of alkynes to cis alkenes ... 

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 ... 

Precipitation of Actinide Complex Anions with Cobalt(III) or Chromium(III) Complex Cations... 

Fig. 13.91. Monoazo cationic (1), disperse (2-4), 1 1 chromium complexed (5), and mordant (6) dyes prepared via an A -> E process.

The chromium(II) cation [Cr(terpy)2] is conveniently prepared by the interaction of aqueous chromium(II) solutions with excess terpy, and may be isolated as its perchlorate or iodide salt 239,255,299). The magnetic properties of [Cr(terpy)2][C104]2 have been recorded over the temperature range 20-300 K the complex is low spin its exhibits a nearly linear temperature dependence over this range 299). The complex may be reduced electrochemically (255) or chemically 239). Herzog and Aul have isolated the various [Cr(terpy)2]" complexes that may be obtained by chemical reduction [Cr(terpy)2]l2 (red-brown, = 2.80 BM), [Cr(terpy)2]I (wine-red, = 1.85 BM), and [Cr(terpy)2] (green, /r ff = 0.63 BM) 239). 

As with the analogous chromium complexes (420), anions (R ) such as hydride, cyanide, alkoxide, etc., add to cyclohexa- or cyclohepta-dienyliron complexes to form the sub.stituted species of structure (107) (299). Reduction of the cycloheptadienyliron cation with zinc dust... 

The sesquioxide, Cr Oa, containing trivalent chromium, is an amphoteric oxide. It yields chromic salts, such as chromic chloride, CrCla, and sulphate, Cr2(S04)a, which are very stable and show great similarity to the ferric salts and to salts of aluminium as, for example, in the formation of alums. Since, however, chromic oxide functions as a weaker base than chromous oxide, the latter having a lower oxygen content, the chromic salts are more liable to hydrolysis than the chromous salts. This is well marked in the case of the chlorides. Again, in spite of the stability of chromic salts, only a slight tendency to form simple Cr " ions is exhibited, whilst complex ions are formed much more readily, not only complex anions, as in the case of iron and aluminium, but also complex cations, as in the extensive chromammine series. In this respect chromium resembles cobalt and platinum. 

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