Thermodynamic considerations of complex formation an introduction

A detailed discussion of the thermodynamics of complex formation in aqueous solution lies beyond the scope of 

Another source of increase in entropy is important when we are dealing with comparable uncharged ligands (e.g. NH3 and H2NCH2CH2NH2), polydentate ligands form more stable complexes than monodentate ones. 

The number of donor atoms through which a ligand coordinates to a metal ion is defined as the denticity of the ligand. A monodentate ligand possesses one donor atom (e.g. NH3), a bidentate ligand two (e.g. [acac] ) and so on. In general, a ligand with more than one donor atom is termed poly dentate. 

We now compare the stability of complexes framed between a given metal ion and related monodentate and bidentate Ugands, and address the so-called chelate effect. In order to make meaningful comparisrais, it is important to choose appropriate Ugands. An NH3 molecule is an approximate (but not perfect) model for half of the Ugand en. Equations 7.76-7.78 show equiUbria for the displace ment of pairs of NH3 Ugands in [Ni(OH2)6 2n(NH3)2 ] 

For each ligand displacement, AG° is negative and these data (or the values of log K) illustrate that the formation of each chelated complex is thermodynamically more favourable than the formation of the corresponding ammine complex. This phenomenon is called the chelate effect and is a general observation. 

A detailed discussion of the thermodynamics of complex formation in aqueous solution hes beyond the scope of this book, but we discuss briefly entropy changes that accompany the formation of coordination compounds in solution, and the so-called chelate ejfect. In Chapter 21, we look further at the thermodynamics of complex formation. 

VIA Fig. 7.12 This modelled structure of a complex [M(en)3] illustrates that the ligand en coordinates to give a puckered chelate ring. Colour code M, green N, blue C, grey. 

We now compare the stability of complexes formed between a given metal ion and related monodentate and bidentate ligands, and address the so-called chelate effect. 

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The mechanisms of surface chemical reactions represent a problem in coordination chemistry, which is the study of complexes, molecular units comprising a central group surrounded by other atoms in close association. This book is principally an introduction to the interpretation of surface phenomena in soils from the point of view of coordination chemistry. Therefore the basic concept to be discussed is the surface functional group, the central moiety in surface complexes, whose formation provides the most important mechanism of adsorption by the solid phases in soils. No detailed consideration of adsorption isotherm equations or the thermodynamic theory of ion exchange is presented, except insofar as their tenuous relation with surface coordination chemistry is to be illustrated. The discussion in this book is intended to be self-contained, but a previous exposure to soil physical chemistry, soil mineralogy, and the fundamentals of inorganic chemistry will prove helpful. 

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