Chromium complexes polynuclear

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. 

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

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). 

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). 

Hydrolysed, polynuclear Cr complexes are of considerable commercial importance in the dying and tanning industries. In the former the role is that of a mordant to the dye. In leather production it is necessai to treat animat hides to prevent putrefaction and to render them supple when dry. Traditionally, tannin was used, hence the name of the process, but this was superseded towards the end of the nineteenth century by solutions of chromium(lil) sulfate. After soaking in sulfuric acid the hides are impregnated with the Cr solution. This is subsequently made alkaline, when the polynuclear complexes form and bridge... 

We have reported the synthesis and the characterisation of a new series of the polynuclear complex compounds of iron(iii) and chromium(in) where the bridging ligand is squarate dianion, coordinated in a /i-l,3-bis(monodentate) manner. 

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