12th Chemistry Coordination Chemistry

Ti: 3d² 4s² Ti³⁺ : 3d¹ 4s⁰ [Ti(H₂O)₆]³⁺ has one unpaired electron, so it is coloured.

Sc: 3d¹ 4s² Sc³⁺ : 3d⁰ [Sc(H₂O)₆]³⁺ has no unpaired electrons, so it is colourless.

The number of ligand donor atoms bonded to a central metal ion in a complex is called the coordination number of the metal. In other words, the coordination number is equal to the number of σ-bonds between ligands and the central atom. Coordination Number of atoms in BCC = 8

• It does not explain the colour of the complex.
• It considers only the spin magnetic moments and does not consider the other components of magnetic moments.
• It does not provide a quantitative explanation as to why certain complexes are inner orbital complexes and the others are outer orbital complexes for the same metal.

For example: [Fe(CN)₆]⁴⁻ is diamagnetic (low spin) whereas [FeF₆]⁴⁻ is paramagnetic (high spin).

1. Coordination Isomerism:

The interchange of one or more ligands between the cationic and the anionic coordination entities.

[Co(NH₃)₆][Cr(CN)₆] ⇌ [Cr(NH₃)₆][Co(CN)₆] The ligands are reversed.

2. Ionisation Isomerism:

• The exchange of counter ions with one or more ligands in the coordination entity will result in ionisation isomers.
• These isomers will give different ions in solution.

For example: [Pt(en)₂Cl₂]Br₂ ⇌ [Pt(en)₂Br₂]Cl₂
3. Linkage Isomerism:

When an ambidentate ligand is bonded to two different donor atoms by the central metal ion, they are called linkage isomers.

[Cr(H₂O)₅NO₂]Br ⇌ [Cr(H₂O)₅(ONO)]Br
4. Solvate (or) Hydrate Isomerism:

When solvent molecules like water are exchanged by the ligands in the crystal lattice of coordination compounds, it is called solvate isomerism.

[Cr(H₂O)₆]Cl₃ ⇌ [Cr(H₂O)₅Cl]Cl₂·H₂O ⇌ [Cr(H₂O)₄Cl₂]Cl·2H₂O

The crystal field stabilisation energy is defined as the energy difference of electronic configurations in the ligand field (E_LF) and the isotropic field/barycentre (E_iso). CFSE (ΔE_o) = E_LF − E_iso

The central atom/ion is the one that occupies the central position in a coordination entity and binds other atoms or groups of atoms (ligands) to itself through a coordinate covalent bond.

1. C₂O₄²⁻ : Oxalato 2. H₂O : Oxidane 3. Cl⁻ : Chlorido