Properties
7.1 Finding Ways to Make Complexes Talk -Investigative Methods
Molecules are not inherently endowed with the capacity to communicate their characteristics. To find out about their structures and properties, we must develop ways to interrogate them efficiently and effectively -we must make them talk, or reveal their character. Working at the atomic and molecular level, this involves the application of an array of chemical and physical methods which in concert can tell us about the compound we have selected for examination. Our first task is straightforward -just what do we want to know? This may seem a simple question, but is not, since the answer will vary with the scientist. One person may simply want to know what elements a compound contains, which relates to the chemical composition; another may want to know what shape the compound takes, which is a far more complex question involving defining what components are involved and how they are assembled overall. More sophisticated questions usually require more sophisticated instrumentation to provide the answers.
It has been suggested that it is easier to make a new molecule than to discover exactly what it is you've made. In synthesis, the difficulty usually lies not in the execution, but in the separation, isolation and identification of products. This is particularly so with metal complexes, where the often large array of options for products, and their capacity to sometimes undergo further reactions in the process of separation, makes life difficult for coordination chemists. Further, species that exist as dominant components in solution may not be the same as the dominant species isolated in the solid state. All this means that defining molecular structure in both solution and the solid state requires a call on a wide range of physical methods for characterization.
The presence of a metal ion in a coordination complex provides a centre of attention for probing the properties of the assembly and for making use of special properties in diverse applications. Unfortunately, the metal also introduces complications that require the use of a range of sophisticated approaches to interrogate the complex and thus determine aspects of both the structure and properties of the complex formed. Our focus in this chapter will be restricted to a few modest techniques; in particular, ones that interact with our limited models of bonding that have been developed. Thus we will probe electronic spectroscopy and magnetic properties in part and briefly examine a few other key techniques, while leaving many other aspects untouched. However, it is inappropriate to explore these few methods in ignorance of others, so we will begin by at least identifying methods that may be of use to the coordination chemist.