Ligand substitution reactions chromium

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. 

Non-Marcusian linear free energy relationships (if I may again be permitted that barbarism) provide direct evidence for this type of potential surface in octahedral ligand substitution reactions. Both dissociative (e.g., the chloropentaamine of cobalt(III)) and associative systems (e.g., chloropentaaquo chromium(III)) may have values of slopes for the linear free energy relationships indicating non-Marcusian behavior. 

The reactivity of Cr complexes is marked by very slow Ligand Substitution reactions, resulting in unusual configurational stability. Many chiral complexes have been resolved. There has been a recent resurgence of interest in organochrominm(III) complexes, owing to the importance of chromium in catalytic processes see Chromium Organometallic Chemistry). 

The ligand substitution reactions of pentaaquachromium(III) alkyl complexes are unusually rapid compared to substitution on most chromium(III) complexes, and this is attributed to the trans-labilizing effect of the alkyl group/ Thus the substitution (41) falls into the SF time scale and is characterized by the rate constants in Table 9.8, and it is observed that relationship (42) holds for the... 

Two of the papers presented at the Fifth International Conference on Non-aqueous Solvents are of direct relevance to this Report. They deal with solvent effects on kinetics, in the areas of ligand substitution reactions at labile centres, and of preferential solvation in such systems." Another review on preferential solvation and its consequences deals primarily with chromium(iii) complexes, such as the [Cr(NCS)6] anion, in binary aqueous mixtures, but also mentions other groups of inorganic substrates such as low-spin iron(ii) complexes. A short article on the effectiveness of a solvent in catalysis considers such topics as affinities for nf-electrons and polarization potentials. ... 

Monstad, L. Monstad, G. Mechanism of Thermal and Photochemical Ligand Substitution Reactions of Chromium(III) and other Octahedral Metal Complexes, Coord. Chem. Revs. 1989,94,109-150. 

Aryl- and alkenylcarbene complexes are known to react with alkynes through a [3C+2S+1C0] cycloaddition reaction to produce benzannulated compounds. This reaction, known as the Dotz reaction , is widely reviewed in Chap. Chromium-Templated Benzannulation Reactions , p. 123 of this book. However, simple alkyl-substituted carbene complexes react with excess of an alkyne (or with diynes) to produce a different benzannulated product which incorporates in its structure two molecules of the alkyne, a carbon monoxide ligand and the carbene carbon [128]. As referred to before, this [2S+2SH-1C+1C0] cycloaddition reaction can be carried out with diyne derivatives, showing these reactions give better yields than the corresponding intermolecular version (Scheme 80). 

It will not have escaped the reader s attention that the kinetically inert complexes are those of (chromium(iii)) or low-spin d (cobalt(iii), rhodium(iii) or iridium(iii)). Attempts to rationalize this have been made in terms of ligand-field effects, as we now discuss. Note, however, that remarkably little is known about the nature of the transition state for most substitution reactions. Fortunately, the outcome of the approach we summarize is unchanged whether the mechanism is associative or dissociative. 

In order to gain more control over this reaction, chromium salphen dimers were synthesized. The synthetic route was developed in such a manner that the bridging length between the two salphen units can easily be varied and that the synthesis of heteronuclear metal complexes is possible. Since the ligand substitution pattern is highly important for the activity of the catalyst as well as the characteristics of produced polymer, an analogous monomeric Cr(lll) complex was synthesized for comparison [102] (Fig. 35). 

Aromatic ketones arylations, 10, 140 asymmetric hydrogenation, 10, 50 G—H bond alkylation, 10, 214 dialkylzinc additions, 9, 114-115 Aromatic ligands mercuration, 2, 430 in mercury 7t-complexes, 2, 449 /13-77-Aromatic nitriles, preparation, 6, 265 Aromatic nucleophilic substitution reactions, arene chromium tricarbonyls, 5, 234... 

Bis(benzene)chromium(0) is rather easily oxidized, but mixed complexes can be obtained by means of substitution reactions. For example, benzene will replace three CO ligands in chromium hexacarbonyl ... 

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