Substitution, of ligands

Selective substitution of ligands (Cl RS ) at the unique Fe has also been demonstrated (62). 

In 1893 a third important regularity was observed by Nikolai Semenovich Kumakov (1860—1941).107 While investigating the substitution of ligands by thiourea and thioacetamide, Kumakov found that replacement occurs with all the ligands of the cis compound but only with the acid radicals of the trans compound (Scheme 2). Since the two isomers yield different products, this reaction, known as Kumakov s reaction or Kumakov s test, may be used to differentiate cis from trans isomers of dipositive platinum or palladium. Kumakov s classic reaction played a crucial role in Werner s proof of the square planar configuration of Pt11 and in Chemyaev s formulation of the trans effect. 

In our work we tried to combine solid state optical spectroscopy with the method of chemical variation, that is a systematic modification of the complexes (substitution of ligands and metals) as well as changes in the surrounding media (solvents, glasses and crystalline hosts). We are thus able to identify the nature of the excited states, and by studying the above series of related compounds we can discern the principles governing their energies relative to each other. 

Operating within the framework of the Chauvin mechanism, the main consideration for the reaction mechanism is the order of events in terms of addition, loss and substitution of ligands around the ruthenium alkylidene centre. Additionally, there is a need for two pathways (see above), both being first order in diene, one with a first-order dependence on [Ru] and the other (which is inhibited by added Cy3P) with a half-order dependence on [Ru]. From the analysis of the reaction kinetics and the empirical rate equation thus derived, the sequence of elementary steps via two pathways was proposed, one non-dissociative (I) and the other dissociative (II), as shown in Scheme 12.20. The mechanism-derived rate equation is also shown in the scheme and it can thus be seen how the constants A and B relate to elementary forward rate constants and equilibria in the proposed mechanism. 

A wide variety of compounds are known which contain at least one transition metal-tin bond. These derivatives undergo different types of reactions, such as substitution of ligands at the tin or the metal center, photochemical reactions and so on. Selected tin derivatives of such transition metal complexes are shown in Table 11. 

A number of such complexes may be made by oxidizing a solution containing divalent cobalt, together with the appropriate ligand. Other complexes are most easily prepared by substitution of ligands on a complex already formed. The following is a set of typical conversions ... 

A general method of extending and changing the character of metal-metal bonds is by substitution of ligands. While this type of reaction does not generate new metal-metal bonds, it can be of use in modifying the properties and reactivities... 

Tungsten hexahalides are well-known starting materials for the synthesis of WVI or related reduced derivatives. Substitution of ligands or hydrolysis sometimes gave derivatives [WX L6 ] (X, halogen L, any ligand n = 1-5) by incomplete replacement of the halo ligands. 

Synthetic applications of the photochemistry of transition-metal arene complexes are rare, except for the (h -arene)M(CO)j (M = Cr, Mo, W) complexes where photo-substitution of ligands for CO occurs. These complexes also undergo arene exchange upon photolysis ... 

In the opposite sense of polarity of the reactants, ligand A plays the nucleophilic role and the ligand containing the allylic moiety will behave as the electrophile. The substitution of ligand A will therefore correspond to an electrophilic substitution and this process could be named " LC ". 

Yamatera, H. (1958) On the absorption spectrum of bexamminecobalt(III) and related complexes. II. Theoretical study of the Siting and splitting of the first and second bands due to the substitution of ligands. Bull. Chem. Soc. Japan 31, 95. 

In 1958, Yamatera studied shifting and splitting of the first and the second absorption band due to substitution of ligands. Thus, he supported that a broad and unsymmetrical ab >rption band observed in the spectrum of an a-isomer is... 

Synthesis by Substitution Routes. Many of the limitations inherent in the redox route described above can be avoided by preparation of technetium-99m radiopharmaeeutieals by a substitution route, i.e. the classical substitution of ligands onto a pre-reduced and isolated technetium center. Substitution routes allow control over the oxidation state and ligand environment of the technetium product, and permit the synthesis of complexes containing different ligands. By substitution routes it should be possible to prepare series of complexes in which some ligands are held fixed while others are varied in a systematic fashion to affect biological specificity. 

New nthetic routes to technetium-99 complexes and technetium-99m radiopharmaceuticals are based on substitution of ligands onto pre-... 

Mechanistic studies from Semmelhack s group showed that in the first step aryl halide undergoes oxidative addition to the low valent metal complex with formation of organometallic intermediate in a higher oxidation state [1,2]. For example, bis(l,5-cyclooctadienyl)nickel(0), Ni(COD)2, reacts with aryl halide (I) to give arylnickel(II) halide (VIII), which further reacts with another aryl halide molecule to form diarylnickel(IV) halide (IX). Each oxidative addition step includes substitution of ligands at metallic centre. 1,5-Cyclooctadiene (COD) dissociates from the nickel to form a coordinatively unsaturated metallic-centre, which does react with aryl halide. Biaryl II is formed by reductive elimination step from IX with liberation of nickel(II) halide [1,2], Scheme 1. 

Complexes in which ligands are rapidly replaced by others are called labile-, those in which ligand substitution is slow are called inert. To quantify tbis distinction, Henry Taube suggested that those complexes in which the substitution of ligands takes place in less than one minute be called labile. Tbe reaction conditions are specified as a temperature of 25°C and 0.1 M reactants. 

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