Formulas and Names of Coordination Compounds

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  

For the complex ion, neutral ligands are written before anionic ligands, and the formula for the whole ion is placed in brackets. 

Let s apply these rules as we examine the eombinations of ions in several coordination compounds. The whole complex ion may be a cation or an anion. A complex cation has anionic counter ions, and a complex anion has cationic counter ions. It s easy to find the eharge of the central metal ion. For example, in K2[Co(NH3)2Cl4], two K eounter ions balance the charge of the complex anion [Co(NH3)2Cl4] , whieh eontains two NH3 molecules and four CF ions as ligands. The two NH3 are neutral, the four CF have a total charge of 4-, and the entire eomplex ion has a charge of 2-, so the central metal ion must be Co +  

CHAPTER 22 The Transition Elements and Their Coordination Compounds 

Coordination compounds, originally named after the person who first prepared them or from their color, are now named systematically using a set of rules  

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Formulas and names of coordination compounds follow systematic rules. 

Complex Ions Coordination Numbers, Geometries, and Ligands 744 Formulas and Names of Coordination Compounds 745... 

The specific combination of ions in a coordination compound is the key to writing its formula and name. A coordination compound can consist of a complex cation with simple anionic counter ions, a complex anion with simple cationic counter ions, or even a complex cation with complex anion as counter ion. 

The names of coordination compounds can become awesomely long because the identity and number of each type of ligand must be included. In most cases, chemists avoid the problem by using the chemical formula rather than the name itself. For instance, it is much easier to refer to [FeCl(H20)5]+ than to pen-taaquachloroiron(II) ion, its formal name. However, names are sometimes needed, and they can be constructed and interpreted, in simple cases at least, by using the rules set out in Toolbox 16.1. Table 16.4 gives the names of common ligands and their abbreviations, which are used in the formulas of complexes. 

Under general headings such as Cobalt(III) complexes and Ammines, used for grouping coordination complexes of similar types having names considered unsuitable for individual headings, formulas or names of specific compounds are not usually given. Hence it is imperative to consult the Formula Index for entries for specific complexes. 

In formulae and names of salts, double salts and coordination compounds, cations precede anions. Ordering within each of these groups is alphabetical, cf. Section IR-2.15.2. 

Use the Handbook of Chemistry and Physics or a suitable internet site to find rules and examples of nomenclature of coordination compounds, (a) Give the formulas and names of two anionic ligands that are not listed in this chapter, (b) Give the formula and name of a neutral ligand that is not listed in this chapter. 

This overview covers some of the rules for naming simple inorganic compounds. There are additional rules, and some exceptions to these rules. The first part of this overview discusses the rules for deriving a name from a chemical formula. In many cases, the formula may be determined from the name by reversing this process. The second part examines situations in which additional information is needed to generate a formula from the name of a compound. The transition metals present some additional problems therefore, there is a section covering transition metal nomenclature and 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. 

So far, we have identified coordination compounds only by their chemical formulas, but names are also useful for many purposes. Some substances were named before their structures were known. Thus, K3[Fe(CN)g] was called potassium fer-ricyanide, and K4[Fe(CN)g] was potassium ferrocyanide [these are complexes of Fe (ferric) and Fe (ferrous) ions, respectively]. These older names are still used conversationally but systematic names are preferred to avoid ambiguity. The definitive source for the naming of inorganic compounds is Nomenclature of Inorganic Chemistry-IUPAC Recommendations 2005 (N. G. Connelly and T. Damhus, Sr., Eds. Royal Society of Chemistry, 2005). 

Chemical nomenclature deals with names of elements and their combinations. Whereas writing the symbol or the name of an element is straightforward, a choice of which element to write first in the formula and in the name has to be made as soon as an element is associated with one or more other elements to form, for example, a binary compound. The order of citation of elements in formulae and names is based upon the methods outlined below. Furthermore, groups of atoms, such as ions, ligands in coordination compounds and substituent groups in derivatives of parent hydrides, are ordered according to specified rules. 

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. 

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). 

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