Formulae of coordination compounds

As in any field of study, careful attention to nomenclature is required. The rules for names and formulas of coordination compounds are given here, with examples to show their use, but we need to be aware of changes in nomenclature with time. In many cases, the notation used by those who first prepared a compound is retained and expanded in other cases, conflicting rules for names are proposed by different people and only after some time is a standard established. The literature naturally includes papers using all the possible names, and sometimes careful research is necessary to interpret those names that had relatively short lifetimes. 

Rule 16. Abbreviations in formulas. In writing the formulas of coordination compounds, it is frequently customary for convenience to use simple abbreviations consisting of two or more letters for complicated molecules. 

The nomenclature of coordination compounds is described in detail in Chapter IR-9. A brief summary of the construction of formulae of coordination compounds is given here. Many polyatomic compounds may conveniently be treated as coordination compounds for the purpose of constructing a formula. 

Since abbreviations are widely used in the chemical literature, agreement on their use and meaning is desirable. This Section provides guidelines for the selection of ligand abbreviations for application in the formulae of coordination compounds (Section IR-9.2.3.4). Some commonly used ligand abbreviations are listed in Table VII with diagrams of most of the ligands shown in Table VIII. 

IR-9.2.2.4 Charge numbers, oxidation numbers and ionic proportions IR-9.2.3 Formulae of coordination compounds... 

These definitions are then used to develop rules for writing the names and formulae of coordination compounds. The mles allow the composition of coordination compounds to be described in a way that is as unambiguous as possible. The names and formulae provide information about the nature of the central atom, the ligands that are attached to it, and the overall charge on the structure. 

The formulae of coordination compounds and complexes are not enclosed in square brackets [71JEN] (Rule 7.21). Exceptions are made in cases where square brackets are required to distinguish between coordinated and uncoordinated ligands. 

There are three important rules for writing the formulas of coordination compounds, the first two being the same rules for writing the formula of any ionic compound ... 

Writing the lUPAC name given the structural formula of a coordination compound, and vice versa Given the structural formulas of coordination compounds, write the lUPAC names (EXAMPLE 23.1). Given the lUPAC names of complexes, write the stmctural formulas (EXAMPLE 23.2). 

Today, the molecular formulas of coordination compounds are represented in a manner that makes it clearer which groups are part of the coordination sphere and which are not. As indicated in the introduction to this chapter, the metal atom or ion and the ligands coordinated to it are enclosed in brackets. It follows that the cobalt ammonate chlorides can be represented as... 

In names and formulas of coordination compounds, cations come first followed by anions. This is the same order as in simple ionic compounds like NaCl for sodium chloride. For example, the formula [Pt(NH3)4][PtCl4] represents the coordination compound tetraammineplatinum(II) tetrachioridoplati-nate(ll). 

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