Substitution reactions ligands

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), 

For the interchange mechanisms Ij and 1, the reactants are suggested to form a precursor complex in a rapid pre-equilibration prior to the rate-determining interchange of X and Y. 

In ligand substitution reactions, one or mote ligands around a metal ion are replaced by other ligands. In many ways, all inorganic reactions can be classified as either substitution or oxidation-reduction reactions, so Aat substitution reactions represent a major type of inorganic process. Some examples of substitution reactions follow  

The operational approach was first expounded in 1965 in a monograph by Langford and Gray. It is an attempt to classify reaction mechanisms in relation to the type of information that kinetic studies of various types can provide. It delineates what can be said about the mechanism on the basis of the observations from certain types of experiments. The mechanism is classified by two properties, its stoichiometric character and its intimate character. 

The stoichiometric mechanism can be determined from the kinetic behavior of one system. The classifications are as follows  

Dissociative (D) an intermediate of lower coordination number than the reactant can be identified. 

One of the commonest reactions in the chemistry of transition-metal complexes is the replacement of one ligand by another ligand (Fig. 9-3) - a so-called substitution reaction. These reactions proceed at a variety of rates, the half-lives of which may vary from several days for complexes of rhodium(iii) or cobalt(m) to about a microsecond with complexes of titanium(iii). 

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 will not have escaped the reader s attention that the kinetically inert complexes are those of (chromium(iii)) or low-spin d (cobalt(iii), rhodium(iii) or iridium(iii)). Attempts to rationalize this have been made in terms of ligand-field effects, as we now discuss. Note, however, that remarkably little is known about the nature of the transition state for most substitution reactions. Fortunately, the outcome of the approach we summarize is unchanged whether the mechanism is associative or dissociative. 

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The ligand substitution reaction of CH2CI2 in Cp(NO)(Ph3P)Re(ClCH2Cl) " by thiophene and 2,5-dimethylthiophene yields the (S) coordinated complex of type... 

Complex 105 enters the pyrazoleAriphenylphosphine ligand substitution reaction with PPha to give 108 (910M3123). Further reaction with triphenylphosphine and silver tetrafluoroborate gives the heterodinuclear complex 109 (94IC2196). 

Oxidative addition of XY substrates to [IrL2(/x-pz)]2 [La = (CO)2, cod] and [Ir(CD)(PPh3)(/i,-pz)]2 occurs via a two-center, two-electron route toward the iridium-iridium bond-containing species 131 (960M3785 980M2743). Complex 132, which is prepared by the ligand-substitution reaction from [Ir(CO)2 (/x-pz)]2, adds methyl iodide to give 133. 

Square planar complexes of palladium(II) and platinum(II) readily undergo ligand substitution reactions. Those of palladium have been studied less but appear to behave similarly to platinum complexes, though around five orders of magnitude faster (ascribable to the relative weakness of the bonds to palladium). 

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). 

In the same way that we considered two limiting extremes for ligand substitution reactions, so may we distinguish two types of reaction pathway for electron transfer (or redox) reactions, as first put forth by Taube. For redox reactions, the distinction between the two mechanisms is more clearly defined, there being no continuum of reactions which follow pathways intermediate between the extremes. In one pathway, there is no covalently linked intermediate and the electron just hops from one center to the next. This is described as the outer-sphere mechanism (Fig. 9-4). 

Several mono(amidinato) complexes of titanium containing the N,N -bis(trimethylsilyl)benzamidinato ligand have been prepared either by metathe-tical routes or ligand substitution reactions as outlined in Schemes 80 and 81. Trialkoxides are accessible as well as the dimeric trichloride, which can be... 

The reactions of nucleophilic reagents with cationic and uncharged metal carbonyl complexes have received much attention in the past, and it is not surprising that these studies have now been extended to isocyanide metal complexes. Different products in these reactions can arise by three general routes these include ligand substitution, reactions involving attack at a ligand, and reduction of the metal complex. All have been observed in reactions with metal isocyanide complexes. 

In a ligand substitution reaction, two groups must always receive attention. There is a bond to the leaving group to be broken and a bond to the entering group to be formed. The relative importance of these two processes provides a basic dichotomy for the classification of substitutions. If a reaction rate is sensitive to... 

The dipalladium complex underwent facile ligand-substitution reaction with the Pd Pd bond remaining intact under moderate conditions.803 The reaction with two equivalents of bidentate phosphine, dppm (diphenylphosphinomethane), in CD3CN afforded a known complex [Pd2(dppm)2(CF[3CN)2][BF4 ]2804 quantitatively. The reaction with two equivalents of 1,10-phen-anthroline (phen) afforded [Pd2(phen)2(CF[3CN)2][BF4]2.805... 

Three types of ligand substitution reactions are now known for technetium clusters. 

Ligand Substitution Reactions in the Synthesis of 99mTc-Labeled... 

Ligand Substitution Reactions of Hexakis(Thiourea)Technetium(III)... 

The reduction of pertechnetate with concentrated hydrochloric acid finally yields the tetravalent state, and no further reduction to the tervalent state takes place. Therefore, the tervalent technetium complex has usually been synthesized by the reduction of pertechnetate with an appropriate reductant in the presence of the desired ligand. Recently, the synthesis of tervalent technetium complexes with a new starting complex, hexakis(thiourea)technetium(III) chloride or chloropentakis(thiourea)technetium(III) chloride, has been developed. Thus, tris(P-diketonato)technetium(III) complexes (P-diketone acetylacetone, benzoyl-acetone, and 2-thenoyltrifluoroacetone) were synthesized by the ligand substitution reaction on refluxing [TcCl(tu)5]Cl2 with the desired P-diketone in methanol [28]. 

Ligand Substitution Reactions of Tc(V) Complexes in Nonaqueous Solvents... 

In inert systems such as technetium and rhenium, ligand substitution reactions-including solvolysis-proceed under virtually irreversible conditions. Thus, the nature of the reaction center, the nature of the leaving group, and the nature and position of the other ligands in the complex affect the rates and activation parameters in a complicated manner. Most substitution reactions take place via interchange mechanisms. This is not too surprising when the solvent is water - or water-like - and where, in order to compete with the solvent,... 

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