Chemistry Coordination Compounds: Overview, Questions, Preparation

Coordination compounds are special chemicals in which a central metal atom/ion is surrounded by non-metal species called ligands. Such compounds come in very handy in biological, industrial, and medicinal activities.

Composition and Structure of Coordination Compounds

A coordination compound consists of:

• A Central Metal Atom/Ion – Generally a transition metal such as Fe, Cu, Co, etc.
• Ligands – Molecules or ions that donate the electron pair to form a bond with the central atom (e.g., NH₃, Cl⁻, CN⁻).
• Coordination Numbers – The number of ligand donor atoms that are directly bonded to the central metal.
• Complex Ions – The charged species formed because of the metal-ligand interaction.

Nature of Ligands

Ligands operate on the metal center differently and are categorized accordingly:

• Monodentate Ligands – Donate one pair of electrons (e.g., Cl⁻, H₂O).
• Bidentate Ligands – Donate two pairs of electrons (e.g., ethylenediamine, C₂O₄²⁻).
• Polydentate Ligands – Have several binding sites (e.g., EDTA, hemoglobin).

Nomenclature of Coordination Compounds

The naming should follow the conventions:

• Name the ligands first, then the metal center.
• Prefixes (di-, tri-, tetra-) indicate the number of identical ligands.
• The oxidation state of the metal must be given in Roman numerals.

In the example, [Co(NH₃)₆]Cl₃ is Hexaamminecobalt(III) chloride.

Werner Theory

The Swiss chemist Alfred Werner suggested a structural theory for coordination compounds:

• Metals have two different kinds of valencies - Primary (ionisable, e.g., oxidation state) and Secondary (non-ionisable, coordinate number).
• The secondary valency denotes the number of ligands surrounding the central metal.

Isomerism in Coordination Compounds

These compounds show:

• Structural isomerism - comprising coordination isomerism and linkage isomerism.
• Stereoisomerism - comprising geometrical (cis-trans) and optical isomerism.

Bonding in Coordination Compounds – Crystal Field Theory (CFT)

The Crystal Field Theory explains the ligand complexation interactions with orbital clouds of metals:

Octahedral field splitting by splitting the (d)-orbitals into t₂g (lower energy) and eg (higher energy) orbitals.

Tetrahedral field splitting where the (d)-orbitals are split in an opposite manner.

Electronic configuration influences color, magnetism, and reactivity.

Applications of Coordination compounds

• Biological Application - Hemoglobin (iron complex), chlorophyll (magnesium complex), and vitamin B₁₂ (cobalt complex).
• Medical Applications - Cisplatin, chemotherapy drug containing platinum.
• Industrial Applications - Catalysts in organic reactions, purification of water, extraction.

Coordination compounds hold an essential place in chemistry and life sciences because of their intrinsic bonding and properties.