Coordination Compounds Notes

Introduction

Coordination compounds, also known as complex compounds, are an essential topic in inorganic chemistry, particularly within the NEET syllabus. These compounds consist of a central metal atom or ion surrounded by molecules or anions, known as ligands. Understanding the structure, bonding, properties, and nomenclature of coordination compounds is crucial for solving related problems in NEET Chemistry.

Coordination Compounds: Basic Concepts

Central Metal Atom or Ion

The central metal atom or ion in a coordination compound is typically a transition metal. These metals can form multiple bonds with ligands due to their vacant d-orbitals.

Ligands

Ligands are ions or molecules that donate a pair of electrons to the central metal atom or ion to form a coordinate bond. Ligands can be classified based on several criteria:

• Charge: Neutral (e.g., $NH_3$, $H_2O$) or anionic (e.g., $Cl^-$, $CN^-$).
• Dentate: Monodentate (one donor atom, e.g., $Cl^-$), bidentate (two donor atoms, e.g., $C_2O_4^{2-}$), or polydentate (multiple donor atoms, e.g., EDTA).

Coordination Number

The coordination number is the number of ligand donor atoms bonded to the central metal atom or ion. It determines the geometry of the complex:

• Coordination Number 4: Tetrahedral or square planar
• Coordination Number 6: Octahedral

Coordination Sphere

The coordination sphere includes the central metal atom/ion and its attached ligands, enclosed in square brackets. For example, in $[Cu(NH_3)_4]^{2+}$, the coordination sphere is $[Cu(NH_3)_4]^{2+}$.

Counter Ions

Counter ions balance the charge of the coordination complex and are found outside the coordination sphere. For example, in $[Co(NH_3)_6]Cl_3$, $Cl^-$ are the counter ions.

Nomenclature of Coordination Compounds

Naming coordination compounds involves several steps:

1. Name the cation first: If the complex ion is a cation, name it before the anion.
2. Name the ligands: List ligands in alphabetical order before the metal. Use prefixes (di-, tri-, tetra-) to indicate the number of each type of ligand.
3. Name the central metal: If the complex is an anion, add the suffix '-ate' to the metal name.
4. Oxidation state: Indicate the oxidation state of the metal in Roman numerals within parentheses.

Isomerism in Coordination Compounds

Isomerism is a phenomenon where compounds have the same chemical formula but different structures or spatial arrangements. Coordination compounds exhibit several types of isomerism:

Structural Isomerism

1. Ionization Isomerism: Different ions can be produced in solution.
2. Coordination Isomerism: Exchange of ligands between cationic and anionic parts of the complex.
3. Linkage Isomerism: Ligands can coordinate through different atoms (e.g., $NO_2^-$ can bind through N or O).

Stereoisomerism

1. Geometrical Isomerism: Different spatial arrangements of ligands (e.g., cis- and trans- isomers in square planar and octahedral complexes).
2. Optical Isomerism: Non-superimposable mirror images (enantiomers).

Bonding in Coordination Compounds

Valence Bond Theory (VBT)

VBT explains the formation of coordination compounds through the overlap of metal and ligand orbitals:

• Inner Orbital Complexes: Use inner d-orbitals (d^2sp^3 hybridization).
• Outer Orbital Complexes: Use outer d-orbitals (sp^3d^2 hybridization).

Crystal Field Theory (CFT)

CFT explains the color, magnetism, and stability of coordination compounds by considering the effect of ligands on the d-orbital energies of the central metal ion:

• Octahedral Field: Splits d-orbitals into $t_{2g}$ (lower energy) and $e_g$ (higher energy).
• Tetrahedral Field: Splits d-orbitals into $e$ (lower energy) and $t_2$ (higher energy).

$$ \Delta_0 = 10Dq \quad \text{(Octahedral field splitting energy)} $$

$$ \Delta_t = \frac{4}{9} \Delta_0 \quad \text{(Tetrahedral field splitting energy)} $$

Applications of Coordination Compounds

Coordination compounds have a wide range of applications:

• Biological Systems: Hemoglobin, chlorophyll, and vitamin B12 are coordination compounds.
• Medicinal Chemistry: Cisplatin is used in cancer treatment.
• Industrial Processes: Catalysts in polymerization and hydrogenation reactions.

Summary

Coordination compounds are a vital part of inorganic chemistry, with applications ranging from biology to industry. Understanding their structure, bonding, and properties is crucial for mastering this topic in the NEET syllabus. Pay close attention to nomenclature, isomerism, and bonding theories to excel in related questions.