Inert and Labile Compounds

Many reactions require substitution, replacing one ligand by another. A well-studied class of substitution reactions involves aqueous metal ions ([M(H20)m] ) as reactants. These reactions can produce colored products used to identify metal ions  

While the fate of these reactions is partially governed by the relative bond strengths between Fe(lll) and the incoming and departing ligands, the examination of water exchange reactions. 

TABLE 12.1 Rate Constants for Water Exchange in [MCHjO) ]  

Data from R. B. Jordan, Reaction Mechanisms of Inorganic and OrganometaUic Systems, 3 ed., Oxford (New York), 2007, p. 84. 

But [Co(NH3)5] reacts very slowly and is therefore inert (the activation energy for the above reaction is high). Both thermodynamic and kinetic parameters are necessary to understand chemical behavior. For clarity, complexes can be described as kinetically labile or inert. 

Many synthetic reactions require substitution, or replacing one ligand by another this is particularly true when the starting material is in aqueous solution, where the metal ion is likely to be in the form [M(H20) ,] . Some simpler reactions of this type produce colored products that can be used to identify metal ions  

These reactions, and others like them, are very fast and form species that can undergo a variety of reactions that are also very fast. Addition of HN03(H ), 

NaCl(Cr), H3P04(P04 ), KSCN(SCN ), and NaF(F ) successively to a solution of Fe(N03)3-9 H2O shows this very clearly. The initial solution is yellow because of the presence of [Fe(H20)5(0H)] and other hydrolyzed species containing both water and hydroxide ion. Although the exact species formed in this series depend on solution concentrations, the products in the reactions given here are representative  

Compounds such as these that react rapidly are called labile (la -bil). In many cases, exchange of one ligand for another can take place in the time of mixing the solutions. Taube has suggested a reaction half-life (the time of disappearance of half the initial compound) of one minute or less as the criterion for lability. Compounds that 

Werner studied cobalt(III), chromium(III), platinum(II), and platinum(IV) compounds because they are inert and can be more readily characterized than labile compounds. This tendency has continued, and much of the discussion in this chapter is based on inert compounds because they can be more easily crystallized from solution and their structures determined. Labile compounds have also been studied extensively, but their study requires techniques capable of dealing with very short times (stopped flow or relaxation methods, for example, temperature or pressure jump, nuclear magnetic resonance). 

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