Ligand-centred reactions

The effects of these ligands on the second-order rate constants for the Cu (ligand) catalysed reaction of Ic with 2 are modest In contrast, the effects on IC2 are more pronounced. The aliphatic Oramino acids induce an approximately two-fold reduction of Iv relative to for the Cu" aquo ion. For the square planar coordinated copper ions this effect is expected on the basis of statistics. The bidentate ligands block half the sites on the copper centre. 

Ligand substitution reactions at low-valent four-, five- and six-coordinate transition metal centres. J. A. S. Howell and P. M, Burkinshaw, Chem. Rev., 1983, 83, 557-599 (468). 

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. 

These reactions provide an efficient route to the formation of tin-centred radicals carrying a variety of ligands. Their reactions with a variety of ligands such as alkyl halides, alkenes and 1,2-diones have been investigated55, and are summarized in Scheme 13. 

The key feature of these reactions is the interplay between metal-centred and ligand-centred radical species, as discussed in Chapter 5. To summarise, the process involves generation of a nitrogen-centred radical, which is stabilised by charge transfer from the... 

Tn other studies, reaction of [MnX(TPP)] (X = Nf or OCN ) with iodosylbenzene in hydrocarbon or halocarbon solvents yielded products formulated as yi-oxo dimers, [MnlvX(TPP)]20.678 [Mn,vN3(TPP)]20 has been characterized by X-ray diffraction (see Section 41.5.7.1). Infrared evidence supports678,679 the formulation of the dimers as being metal-centred oxidized products rather than containing porphyrin cation radicals the possibility of ligand-centred versus metal-centred oxidation in higher-valent metallo porphyrins has been discussed by several authors.680-682... 

It is useful to consider the reactions of carbonyl metallates separately, since their reactivity is generally concerned with the nucleophilic metal centre and will be discussed below. Simple ligand substitution reactions have already been discussed above, as have redox processes that provide access to carbonyl metallates through reduction of the metal centre. These redox or ligand addition/elimination processes are in principle no different from those encountered for classical ligands. We will now consider reactions in which the carbonyl ligand itself enters directly into the reaction and emerges transformed. 

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