Coordination Chemistry - SS2 Chemistry Past Questions and Answers - page 3

Nomenclature rules for naming coordination compounds follow the guidelines established by the International Union of Pure and Applied Chemistry (IUPAC). These rules ensure a systematic and standardised approach to naming coordination compounds. The key nomenclature rules for coordination compounds include:

1.    Name the ligands: Ligands are named first, in alphabetical order, followed by the central metal ion. For anionic ligands, the name ends in "-o" (e.g., chloride becomes chloro). Neutral ligands are named as they are (e.g., ammonia remains ammonia).

2.    Use appropriate prefixes: Prefixes are used to indicate the number of ligands of each type. The prefixes "di-", "tri-", "tetra-", etc., are used for ligands with more than one occurrence.

3.    Specify the oxidation state of the metal ion: The oxidation state of the metal ion is indicated by Roman numerals in parentheses after the metal's name. However, for metals that commonly exhibit only one oxidation state, the Roman numerals are typically omitted.

4.    Include counterions: If the coordination compound carries a charge, counterions are included in the name.

Systematic naming is important as it allows for clear communication and identification of coordination compounds, even without prior knowledge of their structures. Examples of nomenclature application include [Co(NH3)6]Cl3 (hexaamminecobalt(III) chloride) and [PtCl2(NH3)2] (diamminedichloroplatinum(II)).

Isomerism in coordination compounds refers to the existence of different compounds with the same molecular formula but different connectivity or spatial arrangements. Several types of isomerism are observed in coordination compounds, including geometric isomerism, linkage isomerism, and optical isomerism.

1.    Geometric Isomerism: Geometric isomerism arises when the ligands cannot freely rotate around the metal-ligand bonds, resulting in different spatial arrangements. The two main types of geometric isomers are cis-trans isomerism and facial-meridional isomerism.

●     Cis-Trans Isomerism: Cis isomers have identical ligands on the same side, while trans isomers have identical ligands on opposite sides of a coordination complex. An example is [Pt(NH3)2Cl2], where cisplatin and transplatin are cis and trans isomers, respectively.

●     Facial-Meridional Isomerism: Facial isomers have three identical ligands in one plane, while meridional isomers have three identical ligands in two planes. An example is [Co(en)3]3+, where facial and meridional isomers have different spatial arrangements of the ethylenediamine (en) ligands.

2.    Linkage Isomerism: Linkage isomerism arises when a ligand can bind to the metal ion through different atoms. This results in the formation of isomers with different connectivity. An example is [Co(NH3)5(NO2)]2+, where nitrite ions can bind to the cobalt ion either through the nitrogen or the oxygen atom.

3.    Optical Isomerism: Optical isomerism, also known as enantiomerism, arises when a coordination compound possesses a chiral centre. Chiral compounds are non-superimposable mirror images of each other. Optical isomers are denoted as "R" and "S" configurations based on the Cahn-Ingold-Prelog priority rules. An example is [Pt(NH3)2Cl2], where the presence of two different ligands creates two optical isomers.

These types of isomerism highlight the diversity and complexity of coordination compounds. Isomerism plays a crucial role in determining the physical properties, reactivity, and biological activity of coordination compounds, making their study essential in various fields, including chemistry, biology, and materials science.