Inorganic Chemistry: Periodic Trends and Properties - SS3 Chemistry Past Questions and Answers - page 4

Complex ions and coordination compounds are crucial aspects of inorganic chemistry that involve the formation of complexes between metal ions and ligands. A complex ion is formed when a metal ion, acting as the central atom, associates with one or more ligands, which are typically neutral molecules or negatively charged ions. Ligands coordinate with the metal ion through their lone pairs, forming coordinate covalent bonds.

One classic example of a complex ion is the tetra amine copper (II) complex, [Cu(NH3)4]2+. Here, the copper(II) ion (Cu2+) coordinates with four ammonia molecules (NH3) as ligands.

The significance of complex ions and coordination compounds lies in various fields:

1.    Biochemistry: In biological systems, metal ions often form coordination compounds with specific ligands, playing vital roles in enzymes, transport, and signalling processes. For example, haemoglobin contains a coordinated iron(II) ion in the heme group, enabling oxygen transport in the blood.

2.    Medicine: Coordination compounds have found application in medicine, particularly in cancer treatment. Platinum-based coordination complexes like cisplatin have proven effective as anticancer drugs.

3.    Industrial Applications: Complex ions play a crucial role in catalysis and industrial processes. For instance, Wilkinson's catalyst, [RhCl(PPh3)3], is used in the hydrogenation of alkenes.

Environmental Chemistry: Coordination compounds have environmental significance, as they can affect the mobility and toxicity of metal ions in natural systems. Chelating agents form stable complexes with metal ions, influencing their bioavailability and potential environmental impact.

Isomerism is a fascinating phenomenon observed in coordination compounds, where different compounds have the same chemical formula but differ in their arrangement of ligands around the central metal ion. Isomerism in coordination compounds can be broadly categorised into two types: structural isomerism and stereoisomerism.

1.    Structural Isomerism:

a.    Ionisation Isomerism: In this type, ligands and counterions exchange places. For example, [Co(NH3)5Cl]Cl2 and [Co(NH3)5Cl2]Cl have the same chemical formula but differ in the arrangement of Cl and Cl2-.

b.    Coordination Isomerism: This occurs when ligands exchange places between the central metal ions in a complex. For instance, [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6] are coordination isomers.

2. Stereoisomerism:

a.    Geometric Isomerism (Cis-Trans Isomerism): It arises when ligands can be arranged differently around a coordination complex. Common in octahedral complexes with bidentate ligands, like [Pt(NH3)2Cl2], which can exist in cis and trans isomers.

b.    Optical Isomerism (Enantiomerism): This type of isomerism arises when a complex lacks a plane of symmetry and has non-superimposable mirror images (enantiomers). Enantiomers rotate plane-polarised light in opposite directions. For example, [Co(en)3]3+ exists as two enantiomers, which are mirror images of each other.

The importance of isomerism in coordination chemistry lies in its significant impact on the physical and chemical properties of coordination compounds. Isomers often exhibit different colours, solubilities, and reactivities. This property is exploited in various applications, such as in pharmaceuticals, where isomers may exhibit varying biological activities. Understanding isomerism is crucial in designing coordination compounds with specific desired properties and functionalities.