Coordination compounds ligand arrangements

Many complexes and coordination compounds exist as isomers, compounds that contain the same numbers of the same atoms but in different arrangements. For example, the ions shown in (13a) and (13b) differ only in the positions of the Cl ligands, but they are distinct species, because they have different physical and chemical properties. Isomerism is of more than academic interest for example, anticancer drugs based on complexes of platinum are active only if they are the correct isomer. The complex needs to have a particular shape to interact with DNA molecules. 

Isomers are substances that have the same number and kinds of atoms, but arranged differently. Constitutional (structural) isomers, as applied to coordination compounds, are isomers whose differences involve having more than a single coordination sphere or different donor atoms on the same ligand. 

It has become customary to classify transition-metal compounds in terms of oxidation state, and then discuss various types of ligand arrangements within that state. Much of the single element chemistry in Comprehensive Coordination Chemistry ( follows this pattern (e.g., Re), but one or two elements (e.g., Co), are discussed in terms of ligand type. Using the latter approach, patterns of reactivity can be specified by considering changes in oxidation state. 

Figure 13 Schematic representation of coordination compounds with rigid monodentate ligands bearing hydrogen bonding groups (e.g. substituted pyridines). Arrangements VII-XVI are representative of 4-pyridine ligands and arrangements XVII-XXI of 3-pyridine ligands.

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