Chromium polynuclear

Munch, D. 1993. Concentration prohles of arsenic, cadmium, chromium, copper, lead, mercury, nickel, zinc, vanadium and polynuclear aromatic hydrocarbons (PAH) in forest soil beside an urban road. Sci. Total Environ. 138 47-55. 

A review of recent advances in chromium chemistry (82) supplements earlier comprehensive reviews of kinetics and mechanisms of substitution in chromium(III) complexes (83). This recent review tabulates kinetic parameters for base hydrolysis of some Cr(III) complexes, mentions mechanisms of formation of polynuclear Cr(III) species, and discusses current views on the question of the mechanism(s) of such reactions. It seems that both CB (conjugate base) and SVj2 mechanisms operate, depending on the situation. The important role played by ionpairing in base hydrolysis of macrocyclic complexes of chromium(III) has been stressed. This is evidenced by the observed order, greater... 

The use of /r-hydroxo or ju-alkoxo bridged polynuclear complexes of chromium, molybdenum, tungsten, or rhenium in this route leads to the formation of monomeric bis(NHC) complexes, to the elimination of hydrogen, and to the partial oxidation of the metal [Eq.(ll)]. Chelating and nonchelating imidazolium salts as well as benzimidazolium and tetrazolium salts can be used. 

There are many examples today of polynuclear metal complexes yielding a parent ion in the mass spectra, but initially their observation for high molecular weight compounds, such as di[bis(pentafluorophenyl)-phosphidoirontricarbonyl] (I), having a molecular ion of mje 1010 (90) was considered unusual, as were the binuclear chromium complexes (II)... 

Valency Isomerism.—Finally, in the polynuclear compounds occur cases of valency isomerism. For example, the decammino-ol-diehromic salts or rliodo-chromium salts, [(NH3)5Cr.OH.Cr(NH3)5]R5, are isomeric with the decammino-hydroxonium chromic salts or erythro-chromic salts, [(NH3)5Cr.O.Cr(NH3)5]R4. The rhodo-salts are red and... 

B. Chromium(III) Alkyl Compounds Polynuclear Chromium(III) Complexes Polyaminocarboxylic Ligands... 

Complexes of adenine (10 Ade) have been isolated by mixing the ligand in 2-methoxy-ethanol with chromium(II) halides in butanol. The low magnetic moments (Table 16)104 indicate dinuclear species [Cr2X2(Ade)4+] (Section 35.3.5.5), or polynuclear structures with bidentate adenine. 

The coordination chemistry of chromium(III) was first seriously investigated by Pfeiffer at the turn of the century in many ways his studies parallel the work of Werner on cobalt(III). The complexes of chromium(III) are almost exclusively six-coordinate with an octahedral disposition of the ligands. Many are monomeric ((Jeff 3.6 BM), although hydroxy-bridged and other polynuclear complexes are known in which spins on neighbouring chromiums are coupled. 

In this section the methods which have been used to gain structural information are briefly summarized. The term structure is used in this context in its broadest sense, including more qualitative observations concerning the skeleton of the bridging atoms. As a general rule, the hydroxo-bridged polynuclear complexes of chromium(III) and cobalt(III) can be isolated as well-defined crystalline salts and it is therefore quite natural that single-crystal X-ray structure analysis has... 

Only a very few polynuclear complexes containing more than two chromium(III) centers have been studied so far. However, magnetochemical and inelastic neutron scattering studies, heat capacity measurements, and emission spectroscopy have been reported for various tetranuclear species (40,142 151). Two review articles dealing with the spectroscopic and magnetic properties of chromium(III) oligomers have recently appeared (127, 128). 

Polynuclear chromium(III) ethylenediamine complexes have been synthesized by methods similar to those applied for the ammine systems by using the combined catalytic effect of chromium(II) and charcoal on aqueous ethylenediamine buffer solutions (pH -8) of chromium(III) (40, 42, 60). As mentioned above, the use of catalysts is important when equilibration between the mononuclear species is required, but it is unnecessary when the aim is to produce polynuclear species. In fact, identical polynuclear species are formed in approximately the same ratio when buffered chromium(III) solutions ([Cr] =0.1 M, [en = 0., i M) without catalyst are kept for a few days at 40 50 C (40). 

One way to overcome the above problem would be to suppress hydrolysis of the amine ligands by working with an appropriate amine buffer medium. This strategy has been used with great success by Andersen et al. to obtain quantitative equilibrium data for the formation of mononuclear amine complexes (195, 196). Andersen et al. have also studied the formation of polynuclear complexes under similar conditions, but equilibrium was not attained with respect to these species (40, 42, 60, 87). The fact, however, that both thermal hydrolysis and charcoal/chromium(II)-catalyzed hydrolysis in such an amine buffer medium give the same polynuclear species in almost identical ratios would seem to indicate that some degree of equilibration had been achieved. It therefore seems likely that these methods could, in principle, be modified so as to also be applicable for equilibrium studies. Quite a different approach would be to study complexes with macrocy-clic amines such as cyclam, which are known to have a reduced tendency to hydrolysis. However, such systems have not as yet been studied in detail. 

Hydrolysis of polynuclear hydroxo-bridged chromium (III) complexes in concentrated solutions of strong acid yields the corresponding mononuclear species. Such cleavage reactions are fast in comparison with the hydrolysis in dilute acid and proceed with retention of configuration of the mononuclear entities. A few representative examples are shown in Eqs. (46)-(49) (40, 42,161, 252). 

Analysis of the products of these cleavage reactions has often served as proof of the structures of the polynuclear species. Cleavage of hydroxo-bridged complexes of nuclearity higher than two will in most cases yield at least two different mononuclear species. Identification of these species and determination of the relative ratio in which they are formed reduce the number of possible bridged skeletons greatly, and the studies of polynuclear ammine and amine chromium(III) made by Andersen et al. (mentioned in Section IV) provide many examples of this, one of which is shown in Eq. (48) above (see also Section II,A). 

The cleavage of polynuclear hydroxo-bridged rhodium(III) and iridium(III) complexes into the corresponding mononuclear fragments has been reported in only a few instances, but the well-established tendency of mononuclear complexes of these metal ions to undergo substitution reactions with retention of configuration indicates the possibility of analytical and synthetic applications such as described above for chromium (III). 

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