Stability of coordination compounds

The complex [BF3(N(SiH3)3)] can be prepared and stored at low temperatures ( — 80 °C) since the decomposition then proceeds very slowly—at this temperature the complex is kinetically fairly stable. At room temperature the complex is kinetically unstable and the rate of decomposition is much greater. This is the key distinction made in Chapter 4 between kinetically inert and kinetically labile complexes. There it was pointed out that the species which crystallizes from a solution of a mixture of related labile complexes depends not only on the cation and ligand concentration but also on the solvent and crystallization temperature. Although it may be a relatively minor component in the solution, the least soluble complex is probably the one which crystallizes. In the solution there is a series of equilibria such that. 

By convention the displaced water is ignored since its concentration is essentially constant. The overall stability (or formation) constant can clearly be expressed in terms of the stepwise constants  

These are thermodynamic constants which relate to the system when it has reached equilibrium, and must be distinguished from any considerations of kinetic lability or inertness which refer to the speed with which that equilibrium is attained. 

A vast amount of data has been accumulated from which a number of generalizations can be inferred concerning the factors which determine the stabilities of such complexes. Some of these are as follows  

If Ca is added it pushes the equilibrium to the left by bonding preferentially to H2O, whereas Zn with its partial b character (p. 1206), prefers the heavier CF and so pushes the equilibrium to the right. 

It seems that, as suggested by Ahrland et alP in 1958, this distinction can be explained at least partly on the basis that class-a acceptors are the 

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The thermodynamic stability of coordination compounds is relatively easy to determine, and provides us with a valuable pool of data from which we may assess the importance of ligand-field and other effects upon the overall properties of transition-metal compounds. The bulk of this chapter will be concerned with the thermodynamic stability of transition-metal compounds, but we will briefly consider kinetic factors at the close. 

For example, phosphines (RjP) and thioethers (R2S) have a much greater tendency to coordinate with Hg24, Pd24, and Pt2+, but ammonia, amines (R N), water, and fluoride ions prefer Be24, Ti4+, and Co3+. Such a classification has proved very useful in accounting for and predicting the stability of coordination compounds. 

Kumok, V.N. Regularities in Stability of Coordination Compounds in Solutions. University of Tomsk Tomsk, 1977. 

At the same time, this method has a series of disadvantages. Among them, we note the possibiity of contamination of the final product not only by the excess of one of the reactants [2], but also by complexes of the components of the ligand system. So, to carry out strictly template synthesis experiments, it is necessary to take into account a comparative stability of coordination compounds, obtained on the basis of initial components-precursors and the ligand itself. Not only the thermodynamic characteristics of complex-formation processes should be taken into consideration [326,327], but also the influence of solvolysis processes (especially hydrolysis) and the type of atmosphere (air oxygen). 

FACTORS INFLUENCING THE THERMAL STABILITY OF COORDINATION COMPOUNDS... 

Milicevic, A. and Raos, N. (2006) Estimation of stability of coordination compounds by using topological indices. Polyhedron, 25, 2800-2808. 

In Chapter 5 the stability of coordination compounds was discussed in the present chapter, reaction rate or lability is considered. Note that these terms relate to different phenomena. The stability of a complex depends on the difference in free energy between reactants and products. A stable compound will have a lower free energy than possible products. The lability of a compound depends on the difference in free energy between the reactants and the activated complex if this activation energy is large, the reaction will be slow. 

The thermodynamic stabilities of coordination compounds are typically measured using stability or formation constants, as shown in Equations(l5.l)-(l5.4) for Cu(NH3)4+. The tetraaquacopper(ll) cation is used as the starting material in Equation (15.1) because the hydration enthalpy Is so negative that most metal ions cannot exist as naked cations in aqueous solution. It is not always possible for the stepwise constants to be measured individually, so typically only the overall formation constant is reported, where n is the number of ligands attached to the metal ion. If the stepwise stability constants do happen to be known, then the overall constant can be determined from the product of each individual formation constant. The stepwise formation constants for coordination compounds usually decrease in magnitude as the value of n increases. This is an entropic effect that has to do with the number of available substitutions. Thus, for example, addition of NH3 to [Cu(H20)4] " in Equation (15.1) has four possible positions available for substitution, whereas addition of NH3 to [Cu(NH3)3(H20)] in Equation (15.4) has only one possible position available for substitution. 

Studies of the coordination compounds of boron, especially of its halides, alkyls and hydrides, have contributed substantially to our understanding of the factors which influence the stability of coordination compounds in general. The factors which particularly concern boron complexes are inductive and steric efi ects and reorganization energies, as explained below. 

Referred the reader to additional tools for approaching the nature of the M-L interactions and the related topic of the overall stability of coordination compounds found in Chapter 6... 

In view of what we have suggested about the group Va and Via oxides providing a possible means for the removal of steric factors through this part of the Periodic Table, it is disappointing to have very little information on stabilities of coordination compounds of these oxides with Lewis acids although information of this type should be forthcoming in the near future in view of recent activity in the... 

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