Chromium complexes spectroscopic techniques

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

Complexes of lapachol with diverse metals like copper (II), iron (II), iron (III), chromium (III), aluminium (IE), yttrium (El), samarium (IE), gadolinium (IE), and dysprosium (IE) have been investigated [141-144]. Direct electrochemical synthesis of some metal derivatives of lapachol have been carried out [145]. Several P-lapachone hydrazo compounds were synthesized and characterized using spectroscopic techniques including X-ray analyses [146]. Selective aromatic reduction in pyranonaphthoquinones has also been reported [147-148]. 

The biochemical mechanism that allows chromium to potentiate the actions of insulin receptors on cell membranes has been intensively investigated. It is now suggested that a low molecular weight intracellular octapeptide (LMWCr), also known as chromodulin, binds Cr and enhances the response of insulm receptors. Chromodufin binds four Cr ions and then locates on cell membranes near to the site of insulin receptors. The structure of chromodulin has been examined by a variety of advanced spectroscopic techniques and the complex shown to possess a unique type of multmuclear assembly, with the chromium centers having an octahedral coordination with oxygen-based ligands. ... 

Arenes form ir-complexes with all the transition metals. Many ir-arene complexes have been isolated, structurally characterized, and studied by a plethora of spectroscopic techniques. The first T -arene complexes were prepared by Hein in 1919, but the structures of these compoimds were not fully recognized until 1954. The prototype "sandwich" complex, bis(benzene)chromium, Cr(CjH )2, was prepared by E. 0. Fischer in 1955. ... 

Again, making use of a variety of spectroscopic techniques, the structure and stereodynamics of the M(CO)5[P(o-MeC5H4 )3] complexes (M = Cr, Mo, W) have been reported. Szczecinski has released details of an application of scalar C and couplings between carbonyl carbon atoms and aromatic fluorine atoms to investigate the conformation of chelated phosphitodicarbonylchromium and dicarbonyl(phosphine) chromium complexes of fluorobenzenes. 

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