Inert metal complexes ligands

Bipyridines were efficiently used in supramolecular chemistry [104], Since the molecule is symmetric no directed coupling procedure is possible. In addition, 2,2 6/,2//-terpyridine ligands can lead to several metal complexes, usually bis-complexes having octahedral coordination geometries [105,106], Lifetimes of the metal-polymeric ligand depend to a great extent on the metal ion used. Highly labile complexes as well as inert metal complexes have been reported. The latter case is very important since the complexes can be treated as conventional polymers, while the supramolecular interaction remains present as a dormant switch. 

A number of reactions of inert metal complexes are nicely explained by a preliminary deprotonation of the NFI2 ligand. These include base hydrolysis (equation 7), NFI — ND exchange,s-NFI racemization, and some N-C bond formation reactions (equation 12). In every case, the rate of the reaction is proportional to the OH concentration and, as mentioned earlier (see Section 1), in favorable cases the deprotonated species can be isolated and characterized. 

Lability for at least one coordination site has been adequately demonstrated in the traditionally inert metal complexes in recent decades. Induction of lability by chemical reactions on a normally poor leaving group, by metal ions, protons, and base and by polyelectrolytes and micelles, offer opportunities in specific circumstances. Largely during the last decade, however, a number of ligands have appeared that are poor nucleophiles and inherently labile. Such molecules, of which tri-fluoromethanesulfonate is the most extensively studied, are univer-... 

The inert metal complexes such as Cr(III) (CFSE = -1.2A) and Co(III) (CFSE = -2.4A) have large crystal field stabilisation energies. In the case of Co(lII) with six nitrogen donors the CFSE is ca. 250 kJ mof. Energies of this magnitude compare with the values of AH for ligand exchange processes, thus for the reaction. 

This chapter deals with substitution reactions, including aquation, base hydrolysis, formation, and ligand exchange and replacement, and isomerization of inert metal complexes in which the metal has a co-ordination number of five or more. In fact the great majority of the references reported are concerned with octahedral complexes references to complexes of other coordination numbers have been collected together at the end of this chapter (Section 10). 

Ligand Exchange Reactions of Inert-Metal Complexes—Coordination Numbers 4 and 5... 

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. 

The pendant type macromolecule-metal complex is obtained by a substitution reaction between a polymer ligand and an inert metal complex, as presented in Eq. (6). A polymer ligand is coordinated to the vacant site of... 

As well as phosphorus ligands, heterocyclic carbenes ligands 10 have proven to be interesting donor ligands for stabilization of transition metal complexes (especially palladium) in ionic liquids. The imidazolium cation is usually presumed to be a simple inert component of the solvent system. However, the proton on the carbon atom at position 2 in the imidazolium is acidic and this carbon atom can be depro-tonated by, for example, basic ligands of the metal complex, to form carbenes (Scheme 5.3-2). 

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. 

From the viewpoint of coordination chemistry, a substitution reaction can be defined as a process whereby a ligand in a complex is replaced by another ligand from outside the coordination sphere [1], Substitution reactions by metal complexes have been classified by Saito [2] according to Taube s definition [3] of inertness. Saito classified metal ions into three groups as follows ... 

Electron transfer between metal ions contained in complexes can occur in two different ways, depending on the nature of the metal complexes that are present. If the complexes are inert, electron transfer occurring faster than the substitution processes must occur without breaking the bond between the metal and ligand. Such electron transfers are said to take place by an outer sphere mechanism. Thus, each metal ion remains attached to its original ligands and the electron is transferred through the coordination spheres of the metal ions. 

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