Coordination chemistry ligand substitution reactions

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 ... 

NO is most often a three-electron donor ligand. It can be used as the neutral molecule and in simple coordination chemistry can substitute for CO readily. Usually two NO ligands will displace three CO ligands as in the Fe(CO)5 and Fe(CO)2(NO)2 pair. A common method of producing NO-substituted compounds, however, is not the direct reaction of NO with metal fragments, but rather the use of NO+ salts (BF4 or PF6 are commonly employed) as in Eq. (242).357 For metal carbonylate anions this provides a convenient methodology. Notice that CO is still displaced in these reactions. 

Solvent free methods have been used extensively in supramolecular chemistry, coordination chemistry and the formation of transition metal clusters and polymers. Reactions range from very simple ligand substitution reactions for salts of labile metal ions to more complex procedures, some of which are outlined below. 

Ligand substitution is one of the most characteristic reactions of coordination compounds. If in the early stages of the development of coordination chemistry a ligand substitution reaction served as a synthetic method, then later on, especially after Werner, such reactions were widely employed both to solve structural problems (viz., geometric isomerism), and to elucidate the nature of a trans effect. 

On the coordination chemistry side, ligand substitution on metal complexes in ILs has attracted quite some interest. This is mainly due to the fact that both spectroscopic and catalytic properties are strongly governed by the nature of the ligands and the stability of their bond to a metal center. Begel et al. have studied the role of different ILs on ligand substitution reactions on [Pt(terpy)Cl]+ (terpy = 2,2 6, 2"-terpyridine) with thiourea with stopped-flow techniques. The substitution kinetics show similar trends if compared to conventional solvents with similar polarities. Moreover, much like in conventional solvents, the authors find an associative character of the substitution reaction [205], These results are essentially supporting an earlier study by Weber et al., who found the same behavior [206],... 

Ligand Substitution Reactions. Flash photolysis has been adopted to study the substitution behavior of reactive intermediates in organometallic chemistry. Irradiation of M(CO)6 (M = Cr, Mo, W) in a coordinating solvent (S) produces intermediates of the type M(CO)sS [135], which can undergo rapid solvent displacement by a nucleophile (L) to produce M(CO)5L as shown in Eq. (46). The effect of pressure on such substitution reactions has been studied for a series of M, S, and L, for which the results are summarized in Table 11. 

We shall examine in some detail here one of the most important, simplest and best understood classes of reactions in coordination chemistry, namely substitution. This involves one (or several) ligand(s) departing the coordination sphere to be replaced by one (or several) others, without a change in coordination number and basic shape between the reactant complex and its product. It is usually a clearly observable reaction, at least where the nature of the ligands involved in the substitution are distinctively different, and can often be followed by simple approaches such as observing the change of colour as reaction proceeds. 

Ligand substitution reactions of metal complexes have been the topic of many mechanistic studies in coordination chemistry because of the fundamental role of such reactions in many chemical, biological and catalytic processes. For a general ligand substitution reaction as shown in Eq, (1.1),... 

Alkyl-substituted amines present a persistent challenge in this chemistry, as the nucleophilic nature of these amine substrates result in systems that do not realize catalytic turnover, and consequently, well-characterized examples of zwitterionic products resulting from nucleophilic addition to t-coordinated ethylene are well known [88, 112, 160, 144, 161-164]. These species may result in the stoichiometric formation of ethylated amines upon quenching of the reaction [161, 162], but limited catalytic reactivity has been obtained [88, 163]. Alternatively, ligand substitution reactions can result, whereby a neutral coordinated ethylene unit is displaced by the amine and a new amino complex with no further reactivity is obtained [165]. 

The results presented herein show that the redox reactivity of Mn(III) capped porphyrin is comparable to that previously observed for the unhindered analogues. All charge transfer processes involve a penta-coordinate metal center and, in some instances, involve ligand substitution reactions. The presence of the cap does not impede the generation of the high valent nitrido Mn(V) porphyrin. When compared to the reactivity of sterically unencumbered porphyrins, the presence of the cap actually facilitates the production of the nitrenoid complex, the key intermediate in the nitrogen transfer chemistry leading to aziridines. 

Tin amidinates display a rich coordination chemistry with the metal in both the di- and tetravalent oxidation states. The first results in this area were mainly obtained with N-silylated benzamidinate ligands. Typical reactions are summarized in Scheme 48. A stannylene containing unsymmetrically substituted amidinate ligands, [o-MeC6H4C(NSiMe3)(NPh)]2Sn, has been prepared accordingly and isolated in the form of colorless crystals in 75% yield. ... 

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