Thermodynamic stability, /-block metal

In this article the design, synthesis and d-block metal ion chemistry of some more recent examples of covalently-linked, macrocyclic ligand systems are discussed. The use of macrocyclic rings in such systems is not surprising given that the resulting macrocyclic complexes often exhibit both enhanced kinetic and thermodynamic stabilities and hence tend to retain their integrity under a variety of conditions - a lesson that nature knows well. 

This area continues to provide many new compounds and structures, reflecting the versatility and thermodynamic stability of polysulfide anions. Most of the compounds are described under the appropriate metal elsewhere in the Encyclopedia, some of the most elegant recent developments have been with other p-block elements such as tellurium, bismuth, and antimony. ... 

For metals exhibiting variable oxidation states, the relative thermodynamic stabilities of two ionic halides that contain a common halide ion but differ in the oxidation state of the metal (e.g. AgF and AgF2) can be assessed using Bom Haber cycles. In such a reaction as 17.19, if the increase in ionization energies (e.g. M — M versus M— M +) is approximately offset by the difference in lattice energies of the compounds, the two metal halides will be of about equal stability. This commonly happens with block metal halides. 

Within the voltage limits set by the thermodynamic stability range of the electrolyte, foreign metal electrodes may sometimes be regarded as ideally polarizable or blocking. The metal electrodes must not react with the electrolyte, and for the moment adsorption and underpotential deposition will be neglected. From an electrochemical point of view, this is the simplest type of interface and has furnished much of the information we have about the electrified interface. 

Stabilization of metal d-electrons has been employed previously to explain thermodynamic and kinetic data for 6-coordinate hexa-aqua divalent transition metal complexes from the first-row of the d-block. Kinetic data for the dissociation of one lattice water from M +(H20)6 were analyzed by postulating a 5-coordinate square pyramidal product [i.e., M (H20)5] that was not allowed to distort. The... 

The most stable oxidation state for all lanthanide elements is the +3 state. This primarily arises as a result of the lack of covalent overlap, which stabilizes low and high oxidation states in the d-block metals by the formation of Ji bonds. While some zero-valent complexes are known, only the +2 and -1-4 oxidation states have an extensive chemistry and even this is restricted to a few of the elements. The reasons for the existence of compounds in the -1-4 and -j-2 oxidations states can be found in an analysis of the thermodynamics of their formation and decomposition reactions. For example, while the formation of all LnF4 and LnX2 is favorable with respect to the elements, there are favorable decomposition routes to Ln for the majority of them. As a result, relatively few are known as stable compounds. Thus L11X4 decomposition to L11X3 and X2 is generally favorable, while most UnX2 are unstable with respect to disproportionation to LnXs and Ln. 

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