Labile and Inert Coordination Compounds

Because sjmtheses of coordination compounds frequently involve ligand substitution reactions, an understanding of the relative lability (or its opposite, kinetic inertness) of metal complexes is important in the design of experimental procedures. Geometric isomers of labile complexes pose a special problem in synthesis because facile routes for interconversion may allow isomerization, favoring formation of the thermodynamically more stable isomer. 

The reactions of thermodynamically stable, homoleptic, anionic metal cyanide complexes with CN illustrate the fundamental difference in reactivity between labile and kinetically inert complexes, Eq. 1.1  

In solid-state transformations factors other than electronic and steric effects must be considered also in the evaluation of comparative rates, such as the possible restructuring of a crystal lattice (see Chapter 12). The solid state thermal transformations of the pyridinium salts of [MCI4] 2- (M = Pd, Ft), for example, occur at comparable rates despite the fact that these reactions involve substitution at platinum(II) and palladium(II), Eq. 1.2 and 1.3  

To classify the varying rates of reaction (most commonly with regard to substitution) of coordination compounds, Henry Taube, who received the 1983 Nobel Prize in chemistry for his work in the kinetics of coordination compounds, suggested the terms and inert. If we consider a 0.1 Af aqueous solution, a lahk coordination compound is one that under these circumstances has a half-life of less than a minute. (Recall that half-life is the amount of time required for the concentration of the reactant to decrease to half its initial concentration.) An inert coordination compound, on the other hand, is one with a half-life greater than a minute. 

To illustrate the difference between kinetic lability and thermodynamic stability, we consider some specific examples. Take the familiar Werner complex cation hexa-amminecobalt(III), [Co(NH3)g] it reacts spontaneously with acid. In fact, the equilibrium constant for the reaction corresponding to Equation (5.22) is very large. 

Therefore, we would say that this cation is unstable toward reaction with acid. On the other hand, it takes several days at room temperature to get this reaction to go significantly from left to right, even in 6 MHCL Accordingly, the rate of this reaction is so slow that [Co(NH3)6] must be classified as under these circumstances. This complex cation, then, is unstable (thermodynamically) but inert (kinetically) toward reaction with acid. 

In contrast, tetracyanonickelate(II), [Ni(CN)4] , is exceptionally stable (thermodynamically). The equilibrium constant for its formation, represented in Equation (5.23), is very large, also in the vicinity of 10  

At the same time, this complex anion is labilcy that is, the cyanide ligands in the coordination sphere exchange rapidly with those found free in an aqueous solution. This exchange rate can be measured when carbon-14-labeled cyanide ions are placed in solution with the complex, as represented in Equation (5.24)  

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