Chapter 9 The Coordination Compounds Class 12 Notes | YoLearn.ai

Key Concepts: Must Remember

• Coordination Compounds are compounds in which a central metal atom/ion is bonded to a fixed number of ions or molecules (ligands) by coordinate bonds.
• Werner's Theory: Explains primary (ionizable, satisfies oxidation state) and secondary (non-ionizable, satisfies coordination number) valencies.
• Ligands are Lewis bases that donate electron pairs to the central metal atom/ion. They can be monodentate, bidentate, or polydentate.
• Ambidentate Ligands can coordinate through two different atoms (e.g., SCN- can bond via S or N).
• Chelate Ligands are polydentate ligands that form ring structures with the central metal ion, leading to increased stability (chelate effect).
• IUPAC Naming involves naming ligands alphabetically, followed by the central metal (with suffixes like '-ate' for anionic complexes), and then the oxidation state in Roman numerals.
• Isomerism in coordination compounds includes Structural Isomerism (ionization, hydrate, linkage, coordination) and Stereoisomerism (geometrical and optical).
• Valence Bond Theory (VBT) explains bonding, geometry, and magnetic properties by considering hybridization of metal orbitals.
• Crystal Field Theory (CFT) explains color and magnetic properties by considering the electrostatic interaction between metal d-orbitals and ligand electrons, leading to d-orbital splitting.
• Spectrochemical Series: Arranges ligands based on their ability to cause crystal field splitting (weak field < strong field ligands).

Essential Definitions

Coordination Compound

A compound containing a central metal atom or ion bonded to a group of surrounding molecules or ions (ligands) by coordinate covalent bonds.

Ligand

An ion or molecule capable of donating a pair of electrons to a central metal atom/ion to form a coordinate bond. They act as Lewis bases.

Coordination Number

The total number of ligand donor atoms directly bonded to the central metal atom/ion in a coordination compound.

Central Metal Atom/Ion

The atom or ion (typically a transition metal) that acts as a Lewis acid, accepting electron pairs from ligands.

Chelate Ligand

A polydentate ligand that binds to the central metal atom/ion through two or more donor atoms simultaneously, forming a stable ring-like structure.

Ambidentate Ligand

A monodentate ligand that can coordinate to the central metal atom/ion through two different donor atoms (e.g., NO2- can bond via N or O).

Homoleptic Complex

A complex in which all ligands coordinated to the central metal ion are identical (e.g., [Co(NH3)6]3+).

Crystal Field Splitting Energy (CFSE)

The energy difference between the two sets of d-orbitals (t2g and eg in octahedral fields) after splitting due to interaction with ligands.

Werner's Theory and Fundamental Terminology

Alfred Werner's groundbreaking work laid the foundation for modern coordination chemistry. He proposed that metal ions exhibit two types of valencies: primary valency and secondary valency. The primary valency corresponds to the oxidation state of the central metal ion and is usually ionisable, meaning it can be satisfied by anions. For example, in CoCl3·6NH3, the cobalt has a primary valency of +3, satisfied by three chloride ions. The secondary valency, on the other hand, corresponds to the coordination number of the metal and is non-ionisable. It is satisfied by ligands (molecules or ions) that are directly attached to the metal, forming the coordination sphere. The coordination number dictates the geometry of the complex. The group of ligands and the central metal ion enclosed in a square bracket [] forms the coordination entity. Ions outside the bracket are counter ions and balance the charge of the coordination entity. A key aspect is that the primary valencies are represented by dashed lines in Werner's structures, while secondary valencies are shown by solid lines, indicating their directional nature. This theory successfully explained the existence and properties of many coordination compounds, distinguishing between ions within and outside the coordination sphere.

Worked Examples

• IUPAC Naming of a Complex Give the IUPAC name for [Co(NH3)5Cl]Cl2. 1. Ligands: Pentaammine (NH3, neutral), chloro (Cl-, anionic). 2. Oxidation state of Co: Let it be x. x + 5(0) + (-1) + 2(-1) = 0 => x - 3 = 0 => x = +3. 3. Name: Pentaamminechlorocobalt(III) chloride.
• Determining Hybridization and Magnetism (VBT) Determine the hybridization, geometry, and magnetic nature of [Ni(CN)4]2-. 1. Oxidation state of Ni: x + 4(-1) = -2 => x = +2. So, Ni2+. 2. Electronic configuration of Ni2+: [Ar] 3d8. 3. Ligand: CN- is a strong field ligand. 4. Pairing: In presence of strong field CN-, 3d electrons pair up, leaving one 3d orbital vacant. 5. Hybridization: 3d (1 vacant orbital), 4s (1 vacant orbital), 4p (2 vacant orbitals) participate => dsp2 hybridization. 6. Geometry: Square planar. 7. Magnetic nature: All electrons are paired => Diamagnetic.

Steps to Determine Geometry and Magnetic Property using VBT

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Mastering Isomerism and IUPAC Naming

IUPAC Naming: This is a frequent scoring topic. Pay close attention to the alphabetical order of ligands, the correct prefix for ligand number (di, tri vs bis, tris), the '-ate' suffix for anionic complexes, and correctly determining the oxidation state of the central metal. A common mistake is to forget the Roman numeral for the oxidation state.

Isomerism: You must be able to differentiate between all types of structural and stereoisomers and provide suitable examples. For structural isomerism, linkage, ionization, and hydrate isomers are often confused. For stereoisomerism, practice identifying cis/trans (geometrical) and optical isomers (chiral centers, non-superimposable mirror images), especially for octahedral complexes. Remember that square planar complexes never show optical isomerism due to the presence of a plane of symmetry.

Practice Questions with Solutions

• Q: What is the coordination number and oxidation state of Fe in [Fe(C2O4)3]3-? A: Coordination number = 3 (bidentate C2O4 * 2) = 6. Oxidation state = x + 3(-2) = -3 => x = +3.
• Q: Give the IUPAC name of [Cr(en)3]Cl3. A: Tris(ethane-1,2-diamine)chromium(III) chloride.
• Q: Why is [Ni(CO)4] diamagnetic? A: Ni is in 0 oxidation state (3d84s2). CO is a strong field ligand, causing pairing of 4s electrons into 3d orbitals, resulting in all electrons being paired (3d10). Hence, it is diamagnetic with sp3 hybridization.
• Q: Which type of isomerism is shown by [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br? A: Ionization isomerism, as the counter ions and ligands within the coordination sphere are exchanged.

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