Thermodynamics block metal complexes

Crystal field theory can bring together structures, magnetic properties and electronic properties, and we shall expand upon the last two topics later in the chapter. Trends in CFSEs provide some understanding of thermodynamic and kinetic aspects of J-block metal complexes (see Sections... 

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. 

Cyclam, with its 14-membered ring, has been demonstrated to complex with a very wide range of metal ions (and especially d-block metal ions) and tends to generate metal complexes that are especially kinetically and thermodynamically stable [22], In particular instances it is also known to aid the stabilisation of less common oxidation states such as Ni(III), Cu(III), Ag(II) and Ag(III) [25,26],... 

Takeda, Y., Ikeo, N., and Sakata, N. Thermodynamic study of solvent extraction of 15-crown-5- and 16-crown-6-s-block metal ion complexes and tetraalkylammonium ions with picrate anions into chloroform. Talanta 1991, 38, 1325-1333. 

We have thus far discussed rather generally the nature of restrictive ligand fields. We noted that these restrictions, even when small, could be sufficient to block molecular transformations which are, by themselves, thermodynamically only marginally favorable. Thus, simple olefins coordinated to metals that prefer square planar (with and lower metal complexes) and octahedral coordination should not be expected to undergo [2i-j-2s] cycloaddition. For molecular transformations which are energetically more favorable, the situation is different. Here, the energy provided by the exothermic [2 - -2s] process can reasonably be expected to compensate for attending restrictive field effects. We shall now consider the valence isomerization of quadricyclene (9) to norbomadiene (10). 

The dinuclear metal carboxylates M2(02CR)4 represent an important class of transition-metal complexes with respect to the study of structure and metal-metal interactions. Several new chain compounds of dinuclear metal carboxylates have been isolated and some of them show interesting gas-adsorption properties, giving a new aspect of chain complexes. Takamizawa and coworkers [149] presented evidence for a thermodynamic correlation between the phase transition and gas adsorption for a copper complex which was constructed by van der Waals interactions among chain-building blocks. 

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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