The Foundation of Coordination Chemistry
Before we travel too far, however it may be valuable to look back to the beginnings of coordination chemistry. While examples of coordination compounds were slowly developed during the nineteenth century, it wasn't until the twentieth century that the nature of these materials was understood. They were at a very early stage named 'complex compounds' a reflection of their unexplained structures, and we still call them 'complexes' today. Around the beginning of the twentieth century, the wealth of instrumental methods we tend to take for granted today simply didn't exist. Chemists employed chemical tests, including elemental analyses, to probe formulation and structure, augmented by limited physical measurements such as solubility and conductivity in solution. Analyses defined the components, but not their structure. As a consequence, it became usual to represent
them in a simple way as, for example, CoCl3-6NH3 for Tassaert's pioneering complex that we now know as the ionic octahedral cobalt(III) compound [Co(NH3)6]Cl3.
Simple tests of halide-containing compounds, involving gravimetric analysis of the amount of silver halide precipitated upon addition of silver ion to a solution and comparison with the known total amount of halide ion present from a more robust microanalysis method, identified the presence in some cases of both reactive and unreactive halide. For example, CoCl3-6NH3 and IrCl3.3NH3 precipitated three and zero chlorides respectively on addition of silver ion, meaning only one type of halide was present in each, but of different reactivity. Other species such as CoCl3-4NH3 precipitated only one of the three halides readily, so that there were clearly two types of chloride present in the one compound. It was surmised that one type is held firmly in the complex and so is unavailable, whereas the other is readily ac- cessed and behaves more like chloride ion in ionic sodium chloride. We now know these two classes as coordinated chloride ion (held via a M-Cl coordinate bond) and ionic chloride (present as simply the counter-ion to a positively-charged complex cationic species).
The availability of equipment to measure molar conductivity of solutions was turned to good use. It is interesting to note that coordination chemists still make use of physical methods heavily in their quest to assign structures - it's just that the extent and sophisti- cation of instrumentation has grown enormously in a century. What conductivity could tell the early coordination chemist was some further information about the apparently ionic species inferred to exist through the silver ion precipitation reactions. This is best illustrated for a series of platinum(IV) complexes with various amounts of chloride ion and ammonia present (Figure 3.1). From comparison of measured molar conductivity with conductivities of known compounds, the number of ions present in each of the complexes could be deter- mined. We now understand these results in terms of modern formulation of the complexes as octahedral platinum(IV) compounds with coordinated ammonia, where coordinated chloride ions make up any shortfall in the fixed coordination number of six. This leaves in most cases some free ionic chloride ions to balance the charge on the complex cation.
This type of what we now consider simple experiments provided a foundation for coor- dination chemistry as we now know it. What became compelling following the discovery of these intriguing compounds was to develop a theory that would account for these
Figure 3.1 Identifying ionic composition from molar conductivity experiments for a series of platinum (IV) complexes of ammonia and chloride ion.
observations. One of the key aspects relates to shape in these molecules– and it is a wondrously variable area to behold.
