CBSE Class 12 Chemistry Chapter 9 Revision Notes

Chapter 9: Coordination Compounds Revision Notes

• Coordination compounds have a central atom (or cation) that is coordinated to a number of anions or neutral molecules, and they usually retain their identity in both solution and solid state.
• These can be positively, negatively, or neutrally charged species, such as [Co(NH₃)₆]³⁺, [NiCl₄]^2-, [Ni(CO)₄], and so on.
• In 1893, Werner proposed a theory to explain coordination compound structure and bonding.
• According to this theory metals have two types of valencies in coordination compounds: primary valency and secondary valency.
• Primary valencies are ionisable.
• Secondary valencies cannot be ionized.
• This theory was only partially successful, and it failed to explain many aspects of coordination compounds.
• Today, such spatial arrangements are known as coordination polyhedra.
• The ions outside the square brackets are called counter ions, and the species within the square brackets are coordination entities or complexes.
• Isomers are compounds that share the same molecular formula but differ in their structural arrangements.

Types of Isomerism

Source

Magnetic characteristics

• Inner orbital (low spin) complexes are those in which (n-1) d, ns, and np-orbitals are hybridized to form metal hybrid orbitals, such as [Fe(CN)₆]⁴⁺, [CO(NH₃)₆]³⁺, [Cr(NH₃)₆]³⁺, [Fe(CN)₆]²⁺, [Fe(H₂0)₆]²⁺, [(MnCCN)₆]³
• Outer orbital (high spin) complexes, such as [MnF₆]^3-, [FeF₆]^3-, [Ni(NH₃)₆]²⁺, [Ni(H₂0)₆]²⁺, etc., are complexes in which the metal’s hybrid orbitals are formed by hybridization of ns, np, and nd-vacant orbitals.

Assumptions of Crystal Field Theory

• Point charges are assumed for the ligands.
• The interaction between the point charges and the central metal’s electrons is electrostatic.
• In an isolated gaseous metal ion, the 5d-orbitals have the same energy, indicating that they are degenerate.
• A coordination compound’s [MLn] stability is measured in terms of its stability constant.

Drawbacks of Crystal Field Splitting

• Because ligands are assumed to be point charges, anionic ligands should have the greatest splitting effect.
• But anionic ligands are found at the bottom of the spectrochemical series.
• The covalent nature of the bonding between the ligand and the central atom is not taken into account.
• Metal carbonyl bonding: The metal carbon (M – C) bond has both the – and -bond character in metal carbonyl.

Importance of Coordination Compounds and their Applications

• Used in a variety of quantitative and qualitative chemical tests.
• In the extraction of metals such as silver and gold.
• The formation and subsequent decomposition of metal coordination compounds can be used to purify metals such as Ni.
• Chlorophyll, a magnesium coordination compound, is the pigment responsible for photosynthesis in biological systems.
• Haemoglobin is a ferrous coordination compound that serves as an oxygen carrier.
• In medicinal chemistry, an example of chelate therapy.

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