Interactions ion—ligand

The desirability to obtain thermochemical information on ion-ligand interactions in complexes, M2+Ln, involving alkaline earth and transition metal M2+ ions was one of the principle aims which led to the development of apparatus for electrospray generated ions.70 Unfortunately only modest progress has been achieved so far. 

Many of these are substantially non-nucleophilic and unlikely to effect the rate or course of the reaction, although this should always be checked. References 29 to 31 relate some problems in the use of some of these buffers. Occasionally, one of the reactants being used in excess may possess buffer capacity and this obviates the necessity for added buffer. The situation will often arise in the study of complex ion-ligand interactions when either reactant may be involved in an acid-base equilibrium. 

Ab initio Calculations of Ion-Ligand Interaction Energies 2.1 Introduction... 

In a series of papers various semiempirical quantum chemical procedures were examined for their usefulness to describe ion-ligand interactions (for a review see 7)). 

The results were not very successful in several respects wrong geometries, non-realistic interaction energies and extremly overemphasized charge transfer were obtained7). These techniques are thus unsuitable for a reliable investigation of ion-ligand interactions. 

It is also known that geometry optimizations with too small basis sets may lead to non-realistic geometries especially if torsion angles or pyramidal structures are concerned 122>. In any case well-balanced basis sets are absolutely necessary, i.e. the quality of the basis set should be similar for all atoms. A carefully selected small GTO basis set may give reliable results for the ion-ligand interaction energies 31.125-i26> However, error compensations are at least partly responsible for this success. Therefore an improvement of the basis set may lead to less accurate results 91 >. Non-balanced small basis sets lead to large basis set superposition errors 126 l28>. This error is caused by the fact that in the calculations of the complex the wave functions... 

The importance of polarization functions (like d-orbitals for C, O and N or p-orbitals for H) for the calculation of ion-ligand interaction energies was pointed out by several authors 7J7 19>. Nevertheles many results using well-balanced small basis sets without polarization functions are acceptable if interactions of ligands with alkali or alkaline earth metal ions are considered (see Tables 1 and 3). The importance of the polarization functions in the case of the ammonium-water interaction was demonstrated recently 91>. 

An increase in speed can be achieved by using pseudopotential calculations122151 >. In these type of calculations the inner shell electrons are approximated with a potential and the problem is reduced to a valence electron problem. This technique is very powerful for heavy atoms but the time saving is not more than roughly 50 % for molecules containing first-row atoms only 152). Ion-ligand interactions have been studied with pseudopotential calculations in several cases 153 157). 

A precise theoretical approach should be based on an accurate computation of all the single interaction terms, such as metal ion ligand interactions including electrostatic (ion-ion or ion-multipole and polarization terms) as well as electrostatic and van der Waals terms. Individually... 

There are important new applications of the aza-crown macrocycles for medicinal purposes. The perturbation of metabolic processes based on biological metal ion-ligand coordination can produce a disease or even death. Conversely, undesirable biological processes can be prevented by using certain metal ion-ligand interactions. For example, the weak complexing ability of chlorine to the central platinum ion of ds-platin (61) allows cw-platin to have antitumor activity (Haiduc and Silvestru, 1989). When applied in a biological... 

The lanthanide ion-ligand interaction undergoes changes at thermal excitations of a crystal as well. When the temperature rises, the population of the excited sublevels produces anomalies in the thermal expansion of crystals. Redistribution of electron density in a lanthanide crystal results in significant changes in the vibrational spectrum of a lattice, when variations in temperature occur. This is particularly evident for the anomalous temperature behaviour of elastic constants. 

For ligand nuclei the gyromagnetic ratio Yv is isotropic = yv ajs], W/, and the contribution of the W ion-ligand interaction into the second moment of W ions NMR line is... 

The second and more recent approach which has been used to classify metal ion-ligand interactions is based on the concept of hard and soft acids and bases, often denoted HSAB. Hard metal ions are those which parallel the proton in their attachment to ligands, are small, often of high charge and have no valence shell electrons that are easily distorted or removed. Soft metal ions are large, of low charge or have valence shell electrons which are... 

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