Chromium cations formed

The reaction can be made catalytic by addition of NaaS204 which reduces the bisarene chromium cation formed back to the neutral species 192). 

Chromium(III) forms stable salts with all the common anions and it complexes with virtually any species capable of donating an electron-pair. These complexes may be anionic, cationic, or neutral and, with hardly any exceptions, are hexacoordinate and octahedral, e.g. ... 

Fig. 8 a The crystal is formed of columns of methoxychromate anions and of columns of bis-benzene chromium cations extending parallel to the c-axis. b The anions are apparently linked along the column via a short C-H O interaction (H O 2.381 A, C-H O 173°) between a methyl hydrogen and a chromate oxygen... 

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). 

Bisbenzene chromium reacts with good 77-acceptor Lewis acids to form complexes (CgHg)2Cr L" (L = tetracyanoethylene, trinitrobenzene, -quinone, chloranil) in which electron transfer from the (CgHg)2Cr to the Lewis acid has taken place. The complexes are best described as bisbenzene chromium cation salts of radical anions (765). The crystal structure of one such compound [(MeCgHg)2Cr] (TCNQ) (TCNQ = 7,7,8,8-tetracyanoquinodimethane) has been determined and consists of Stacks of TCNQ anions and bisbenzene chromium cations with interplanar spacings of 3.42 A (577). 

There are a number of organic acids used as ionic carriers for metal ions and amines or amino acids pertraction. The mostly used anionic carrier for metal ions and amino acids is di-2-ethylhexyl phosphoric acid. The carrier is dissolved in the membrane phase as a dimmer, which reacts at the donor phase-membrane interface with amino acid cation, forming an ion pair and releasing a proton. For example, selective speciation of different chromium species (chromate and chromium ions) was achieved by the combination of... 

Finally, between the TOT layers of a smectite, large cationic species that are polymeric or oligomeric hydroxyl metal cations formed by the hydrolysis of metal salts of aluminum, gallium, chromium(III), silicon, titanium(IV), iron(III), and mixtures of them can be inserted by cation exchange, giving the so-called pillared clays... 

H. R. Carvetli and B. E. Curry found that chromium begins to be deposited instantly from a soln. of impure chromic acid at 18° with a current density of about 0 80 amp. per sq. cm. The deposition is not so readily obtained with soln. of purified chromic acid which has a decomposition voltage of 2-31 volts. In all cases, the liquid was coloured brown, and chromic salts were produced the brown precipitate formed at the Cathode is probably Cr(Cr04). It is assumed that sexivalent chromium cations are present in the soln. of chromic acid, and that the increased deposition which occurs when sulphuric acid is present, is due to an increase in the cone, of the sexivalent Cr-cations by a reaction of chromic acid with the sulphuric acid. 

The chemical behavior of Mo should be similar to that of chromium (Cr) in wastewater treatment systems. The two elements can exist in the +3 oxidation state as cations under reducing conditions and tend to form oxides and hydroxides with very low solubilities. In aerated environments, the dominant species are the molybdate, MoO , the oxidized form of [Mo(Vl)] and chromate [Cr(Vl)], which are anions, in contrast to the cationic forms in reducing environments (Magyar et al, 1993 Beaubien et al, 1993). The reversal of charge as a function of oxidation state is expected to change the adsorption characteristics and precipitation reactions of Mo in wastewater systems compared to As and Se. 

The inner two CHD units form a tautomeric anion to which the outer two CHD molecules are connected in their enol form. The resulting [(CHD)4] anion interacts with one bis(7 -benzene)chromium cation via short C-H- - -OC hydrogen-bonds. Finally two such horse-shoe -shaped systems wrapped around the organometallic cation are related by a center of inversion. This results in an overall supramolecular... 

Thus the main result of this experiment was the rational design of a self-assembled crystal structure based on the shape analogy between benzene and bis(t -benzene)chromium. Under similar reaction conditions bis(t7 -toluene)chromium reacts with CHD to afford [Cr( / -C6H5Me)2][(CHD)2], 102 [161b]. In the solid state 102 forms a C-H- -O hydrogen-bonded network in which the bis( 7 -toluene)-chromium cations alternate with clamps formed by the dimeric [(CHD)2] anions. 

In this figure, for zero pH (pH = 0 rev,oxide = the passivation potentials of chromium and iron do not match the standard potentials of the oxides Cr203 and Fc203 listed in Table 6.8. One explanation is that kinetic limitations lead to a higher passivation potential than predicted by thermodynamics. Another reason could be that the listed standard potentials were measured on bulk samples. The extreme thinness of passive films could influence their thermodynamic properties. In addition, their composition does not always correspond to a simple stoichiometry. For example, chromium-iron alloys form passive films containing both iron and chromium cations and their passivation (Figure 6.9) lie between those of iron and chromium. Then again, they exhibit the same pH dependence as the pure metals. 

The cationic forms of Cd, Pb Cr, and Ni were shown to be immobilized in clinoptilolite structure by two mechanisms ion exchange and chemisorption [05M1]. In case of lead and chromium, chemisorption predominates. The contributions of both mechanisms in case of Cd and Ni were equal. The long term kinetics of Cd sorption and desorption by Ca-exchanged clinoptilohte was studied by using isotope exchange technique while maintaining pH at circumneutral values [lOAl]. 

The hydrated chromium(III) cation formed in the hydrolysis solution is strongly adsorbed on the resin, but can be removed from the column with a solution containing 0.5 M perchloric acid and 2.5 M sodium perchlorate. It is identified in the effluent by its typical three-band ultraviolet and visible absorption spectrum with, maxima at 575, 406, and 256 m/i (Fig. 4), and comprises about 11 % of the hydrolysis mixture. 

Elements from the rf-blockform 2-I-, 3-I-, or, in a few cases, 1 -I- or 4-1-cations. Many rf-block elements form two ions of different charges. For example, copper forms H- and 2-1- cations. Iron and chromium each form 2-1- cations as well as 3-1- cations. And vanadium forms 2-I-, 3-I-, and 4-1- cations. 

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