Coordination Compounds Mind Map

Coordination Compounds Mind Map (NCERT Class 12 Chemistry - Chapter 9)

Coordination compounds, also known as complex compounds, are a fascinating class of molecules where a central metal atom or ion is surrounded by a fixed number of ions or molecules. These surrounding species, called ligands, donate electron pairs to the central atom, forming coordinate bonds. This chapter delves into the world of coordination compounds, exploring their structure, bonding, nomenclature, and applications.

1. Werner's Theory of Coordination Compounds:

The foundation for understanding coordination compounds is laid by Alfred Werner's theory. This theory proposes the following key points:

• Central Atom/Ion: A metal atom or ion acts as the central entity in a coordination compound.
• Ligands: Ions or molecules surrounding the central atom/ion and donating electron pairs are called ligands. Common ligands include ammonia (NH3), chloride (Cl-), water (H2O), and cyanide (CN-).
• Coordination Number: The number of ligands bonded to the central atom/ion is termed the coordination number. It depends on the size and charge of the central atom/ion and the size of the ligands.
• Coordination Sphere: The central atom/ion and the ligands directly attached to it form the coordination sphere.
• Primary and Secondary Valences: Werner proposed the concept of primary and secondary valences. Primary valences, satisfied by ionic bonds between the central cation and the anions, are no longer relevant in coordination compounds. Secondary valences, formed by the coordinate bonds between the central atom and ligands, determine the coordination number.

2. Definitions of Some Important Terms:

• Coordination Entity: The central atom/ion along with its ligands is referred to as the coordination entity. It can exist as a cation (positively charged), anion (negatively charged), or neutral molecule. (e.g., [CoCl3(NH3)3])
• Homoleptic and Heteroleptic Compounds: When all ligands surrounding the central atom are identical, the complex is homoleptic. If different ligands are attached, it's a heteroleptic compound.
• Oxidation Number: The oxidation number of the central atom in a coordination compound represents the hypothetical charge it would have if all ligands were removed as anions.

3. Nomenclature of Coordination Compounds:

Naming coordination compounds systematically follows the IUPAC nomenclature rules. Here's a breakdown of the process:

1. Cation Name: If the complex cation is present, name it first.
2. Ligand Names: List the ligand names in alphabetical order. For anionic ligands, their names end with the suffix "-o" (e.g., chloro for Cl-). If there are multiple identical ligands, use prefixes like di-, tri-, etc. (e.g., dichloro for 2Cl-).
3. Central Metal Atom: Mention the central metal atom name. In cases of variable oxidation states, the oxidation number of the metal is written in Roman numerals within parentheses after the name.
4. Overall Charge: If the complex ion is charged, indicate the charge after the entire name in brackets.

4. Isomerism in Coordination Compounds:

Isomers are compounds with the same chemical formula but different arrangements of atoms or ions. Coordination compounds exhibit various types of isomerism:

• Structural Isomerism:
  • Linkage isomerism: Isomers where ligands can bind in different ways. (e.g., [Co(NH3)5(NO2)]2+ vs. [Co(NO2)2(NH3)4]+)
  • Coordination isomerism: Isomers where the arrangement of ligands within the coordination sphere differs. (e.g., [Pt(NH3)2Cl2] vs. [Pt(NH3)(Cl)(NH2CH3)])
• Stereoisomerism:
  • Geometrical isomerism: Isomers with different spatial arrangements of ligands around a central atom in a fixed geometry. (e.g., square planar complexes can have cis- and trans- isomers)
  • Optical isomerism: Isomers that are non-superimposable mirror images, observed in chiral coordination compounds.

5. Bonding in Coordination Compounds:

The formation of coordinate bonds between the central atom and ligands is crucial for understanding coordination compounds. Two main theories explain this bonding:

• Valence Bond Theory: This theory proposes the formation of coordinate bonds by the donation of a lone pair of electrons from a ligand to an empty or partially filled orbital of the central metal atom. The central atom can form multiple coordinate bonds based on the availability of empty orbitals.
• Crystal Field Theory: This theory focuses on the electrostatic interaction between the d-orbitals of the transition metal ion and the surrounding ligands. The splitting of d-orbitals due to the ligand field influences the electronic configuration and stability of the complex.

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