Substitution reactions ligands

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 

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). 

Ligand Substitution Reactions in the Synthesis of 99mTc-Labeled 

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

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

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

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 

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). 

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). 

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 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]. 

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. 

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