Inert metal complexes reactions involving

In volume 7 reactions of metallic salts, complexes and organometallic compounds are covered. Isomerisation and group transfer reactions of inert metal complexes and certain organometallics (not involving a change in oxidation state) are considered first, followed by oxidation-reduction processes (a) between different valency states of the same metallic element (b) between salts of different... 

Some reactions of metal complexes that appear to involve substitution may actually occur without breaking a bond to the metal. Since HCO, is in equilibrium with aqueous COj, reaction (3.47) could be a replacement of OH by HCOj , but 0-labeling studies show that the Co—O bond is retained. The reaction is really electrophilic attack of COj on the coordinated OH Microscopic reversibility requires that the reverse reaction proceeds with C—O bond breaking. This seems to be a general characteristic of CO2 addition and release from such inert metal complexes. For example, it rqrpears that W(CO)j(OH) reacts similarly with COj. ... 

In this section substitution reactions involving the formation (often anation) of inert metal complexes are discussed, with cations arranged in order of their increasing numbers of tZ-electrons. Metal ions with d , d, d or d electronic configurations are usually labile and most references to these systems will be found in Chapter 4. 

A number of complexation reactions of inert-metal complexes with oxo ligands may occur by substitution at the more labile tetrahedral anionic center. The reactions of [CrfHjOlg] " and [Cr(NHj)5(H20)] + with arsenate have been reported. The reaction of the hexa-aqua complex has been measured by stopped flow methods the reaction of the penta-ammine is too fast at 25 °C to quantify with this technique. The substitution mechanism involves three parallel pathways, coupled by labile protonic equilibria involving the As(V) species as shown in Eq. (30). On the basis of the negative activation entropies and the dramatic dependence of the rate on the Cr(III) ligand , an associative mechanism is suggested. 

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. 

It is well known that the comparatively inert Si-H bonds of triorganosilane R3SiH (R = alkyl, aryl) can be activated by transition metal complexes. Chatgililaloglu et al. have used Et3SiH in palladium-catalyzed dehalogenation reactions, which occur with the involvement of free radicals.245... 

The ability of metal ions to catalyze the hydrolysis of peptide bonds has been known for 50 years, while the catalytic effect on the hydrolysis of amino acid esters was highlighted in the 1950s. As Hay and Morris point out in their review,76 the major problem with the kinetically labile systems is determining the nature of the reactive complex in solution. Such problems generally do not arise in the more inert systems and consequently reactions involving Co111 have been the more popular for study. 

Whilst metal-N(peptide) bond formation inhibits hydrolysis of the peptide bond, coordination to O(peptide) has the opposite effect. These differences in reactivity can be readily demonstrated and put to practical use with the inert Co111 complexes. One of the first examples was the reaction of [Co(trien)(H20)(OH)]2+ with peptides to give hydrolysis of the peptide bond at the N-terminal end. The proposed mechanism involving nucleophilic attack by hydroxide at the peptide carbon is shown in Scheme 7.110 Similar selective hydrolyses of N-terminal peptide bonds have since been demonstrated with other Co111 amine complexes and the reaction has been examined as a method for determining the N-terminal amino acid residue in peptides and proteins.1"112... 

In cases where metals or metal ions can contaminate the products, reaction vessels fabricated from inert polymeric materials restrict that possibility. A significant example involved the reaction of maltol with aqueous methylamine to give l,2-dimethyl-3-hydroxypyrid-4-one. The product is a metal chelator employed for the oral treatment of iron overload. Consequently, it is an excellent metal scavenger but must be produced under stringent conditions that preclude metal complexation. Literature conditions involved heating maltol in aqueous methylamine at reflux for 6 h, the product was obtained in 50% yield, but required decolourisation with charcoal135. With the CMR, the optimal reaction time was 1.3 min, and the effluent was immediately diluted with acetone and the near colourless product crystallised from this solvent in 65% yield (Scheme 9.18). A microwave-based batch-wise preparation of 3-hydroxy-2-methylpyrid-4-one from maltol and aqueous ammonia was also developed. 

It has already been stated that chromium complexes of tridentate metallizable azo compounds occupy their position as the single most important class of metal complex dyestuffs because of their high stability. It should be noted, however, that in this context the term stability is not used in the thermodynamic sense but relates to the kinetic inertness of the complexes.25 Octahedral chromium(III) complexes have a tP electronic configuration and the ligand field stabilization energy associated with this is high.26 Ligand replacement reactions involve either a dissociative... 

Although alkynes are highly reactive toward a wide range of transition metals, very few instances of metal-catalyzed reactions of nucleophiles with alkynes are known. This is, in part, because most stable alkyne-metal complexes are inert to nucleophilic attack, while most unstable alkyne-metal complexes tend to oligomerize alkynes faster than anything else. Hence synthetic methodology involving this process is quite limited. 

Despite the fact that carbon dioxide (C02) is used in a great number of industrial applications, it remains a molecule of low reactivity, and methods have still to be identified for its activation. Both thermodynamic and kinetic problems are connected with the reactivity of C02, and few reactions are thermodynamically feasible. A very promising approach to activation is offered by its coordination to transition metal complexes, as both stoichiometric reactions of C-C bond formation and catalytic reactions of C02 are promoted by transition metal systems. Efforts to enhance the yield of hydrogen in water gas-shift (WGS) reactions have also been centered on C02 interactions with transition metal catalysts. The coordination on metal centers lowers the activation energy required in further reactions with suitable reactants involving C02, making it possible to convert this inert molecule into useful products. 

The coordination metal complexes are often air-sensitive, so preparations involving nonaqueous and inert solvents or vapor phase reactions (chemical vapor deposition (CVD), Section A.4.9) are required. The reactions are usually performed between room temperature and a maximum that is limited by the decomposition temperature of the precursor for vapor phase reactions, and the solvent reflux temperature for liquid phase reactions. 

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