Other Ligand Substitutions

A further interesting point comes from the comparison of the in vivo data with the other ligand substitution results in general. The in vivo half-lives listed are comparable with the rate data of the equatorial substitution and might suggest that the mechanisms responsible for the uptake/clearance of the radiopharmaceuticals, be they protein, peptide, or DNA interactions or innersphere redox reactions, might indeed be associated therewith. This is, however, just an observation and surely requires much more research to be well understood. 

Vanadium(III) reacts with O2 and CIO4 and is easily hydrolyzed (pA = 3.0), all important points to consider in studying its reaction kinetics. An 4 mechanism is favored for H2O exchange (Table 4.5) and for other ligand substitutions. This is supported by the activation parameters and the correlation of with the basicity of the entering ligand (Table 8.2). - ... 

Ferrocene itself shows much more chemical stability than cobaltocene and nickelocene many of the chemical reactions of the latter are characterized by a tendency to yield 18-electron products. For example, ferrocene is unreactive toward iodine and rarely participates in reactions in which other ligands substitute for the cyclopentadienyl ligand. However, cobaltocene and nickelocene undergo the following reactions to give 18-electron products. 

Where the rate law has been determined, the reaction is first-order in [M(CO)J and zero-order in [CO]. This implies a D mechanism, since a solvent intermediate is unlikely for the "noncoordinating" solvents. This mechanism also is probable for other ligand substitutions. 

The two protons at C-1 are topologically nonequivalent, since substitution of one produces a product tiiat is stereochemically distinct fiom that produced by substitution of the other. Ligands of this type are termed heterotopic, and, because the products of substitution are enantiomers, the more precise term enantiotopic also applies. If a chiral assembly is generated when a particular ligand is replaced by a new ligand, the original assembly is prochiral. Both C-1 and C-3 of 1,3-propanediol are prochiral centers. 

Numerous carbonyl halides, of which the best known are octahedral compounds of the type [M(C0)4X2] are obtained by the action of halogen on Fe(CO)5, or CO on MX3 (M = Ru, Os). Stepwise substitution of the remaining CO groups is possible by X or other ligands such as N, P and As donors. 

A further factor which must also be taken into consideration from the point of view of the analytical applications of complexes and of complex-formation reactions is the rate of reaction to be analytically useful it is usually required that the reaction be rapid. An important classification of complexes is based upon the rate at which they undergo substitution reactions, and leads to the two groups of labile and inert complexes. The term labile complex is applied to those cases where nucleophilic substitution is complete within the time required for mixing the reagents. Thus, for example, when excess of aqueous ammonia is added to an aqueous solution of copper(II) sulphate, the change in colour from pale to deep blue is instantaneous the rapid replacement of water molecules by ammonia indicates that the Cu(II) ion forms kinetically labile complexes. The term inert is applied to those complexes which undergo slow substitution reactions, i.e. reactions with half-times of the order of hours or even days at room temperature. Thus the Cr(III) ion forms kinetically inert complexes, so that the replacement of water molecules coordinated to Cr(III) by other ligands is a very slow process at room temperature. 

Several other complexes, M(CNBu )jL (L = an activated olefin), have also been reported recently (110). This group of complexes, with the ligands (L) including maleic anhydride, fumaronitrile, and tetracyano-ethylene, arises from isocyanide ligand substitution by the olefin. Less active olefins such as ethylene and diphenylacetylene, and azobenzene did not react. 

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