COMMON LIGANDS IN COORDINATION CHEMISTRY

COMMON LIGANDS IN COORDINATION CHEMISTRY We examine some common geometries found in coordination complexes and how the geometries relate to coordination numbers. 

We illustrate the nomenclature introduced above in an example taken from coordination chemistry. In fact, equilibrium species of interesting complexity are commonly encountered in coordination chemistry and to a large extent coordination chemists have developed the principles of equilibrium studies. Consider the interaction of a metal ion M (e.g. Cu2+) with a bidentate ligand L (e.g. ethylenediamine, en) in aqueous solution. For work in aqueous solution the pH also plays an important role and thus, the proton concentration H (=[ff+]), as well as several differently protonated species, need to be taken into account. Using the nomenclature commonly employed in coordination chemistry, there are three components, M, L, and H. In aqueous solution they interact to form the following species, HL, H2L, ML, Mia, ML3, MLH, MLH1 and OH. (In fact, more species are formed, e.g. ML2H 1, but the above selection will suffice now.) The water molecules are usually not defined as additional components. The concentration of water is constant and its value is taken into the equilibrium constants. 

The cyanide ion, CN , is one of the most common and longest-known ligands see Ligand) in coordination chemistry. 

One of the most versatile classes of ligands in coordination chemistry is that of the /3-diketonates, of which the most common is the acetylacetonate, (acac), Figure 9.1. The coordination chemistry of this ligand first appears in the literature in work by Combes in 1887-1894. Alfred Werner also published on the chemistry of the acac ligand in 1901. The acac ligand is remarkable in that it forms complexes with virtually any metal, including beryllium, lead, aluminum, chromium, platinum, and gadolinium. 

In principle, any molecule or anion with an unshared pair of electrons can act as a Lewis base. In other words, it can donate a lone pair to a metal cation to form a coordinate covalent bond. In practice, a ligand usually contains an atom of one of die more electronegative elements (C, N, O, S, F, Cl, Br, I). Several hundred different ligands are known. Those most commonly encountered in general chemistry are NH3 and HzO molecules and CN , Cl-, and OH- ions. 

Exchange of substituents at the centra] metal can be regarded as ligand-ligand exchange, something common in coordinative chemistry. 

Some commonly used macrocycles. — The number in brackets indicates the size of the ring. The terms ane and ene denote saturated and unsaturated rings respectively. The number of ligating atoms is indicated by a subscript. A trivial name is often used (G. A. Melson in Coordination Chemistry of Macrocyclic Compounds, Ed. G. A. Melson, Plenum, New York, 1979, Chapter 1 Comprehensive Coordination Chemistry, Ed. G. Wilkinson, Pergamon, Oxford, 1987. Several chapters in Vol. 2 (Ligands)). 

The substitution reaction in which a molecule of solvent replaces one of the ligands represents one of the most commonly and conveniently studied processes in coordination chemistry. In labile systems, analysis of the relaxation kinetics in the complex formation studies will, of course, give the rate constants for the solvolysis as well as those for the complex formation. In inert systems... 

Bridging ligands are very common in coordination chemistry and their presence is symbolized by the prefix /u.. Bridging ligands are usually counted as follows. First, we will look at bridging halide. This carries a lone pair, which is donated to the second metal in forming the bridge. An L MC1... 

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