Class 12 Chemistry Important Chapter 9 Coordination Compounds

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Class 12 Chemistry Important Chapter 9 Coordination Compounds

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Coordination Compounds

Chapter: 9

PART – I

IMPORTANT QUESTION AND ANSWER

1. Explain Alfred Werner’s theory of coordination compounds and its main postulates.

Ans: Alfred Werner proposed that coordination compounds have two types of valences: primary and secondary. Primary valence corresponds to the oxidation state and is ionizable (like chloride ions in CoCl₃), while secondary valence equals the coordination number and is non-ionizable (ligands like NH₃ directly bonded to the metal). He suggested that the ligands have definite spatial arrangements around the metal ion forming coordination polyhedra like octahedral, tetrahedral, or square planar. Werner’s theory explained isomerism and bonding in coordination compounds successfully.

2. Define the terms ‘ligand’, ‘coordination number’, and ‘coordination sphere’ with examples.

Ans: (i) Ligand: An ion or molecule that donates a lone pair of electrons to the central metal atom/ion. For example, NH₃, Cl–, H₂O.

(ii) Coordination Number: The number of ligand donor atoms directly attached to the metal. For example, in [Ni(CO)₄], the coordination number is 4.

(iii) Coordination Sphere: The central metal ion along with its ligands enclosed in square brackets, e.g., [Fe(CN)⁶]^4– is the coordination sphere, and ions outside are counter ions.

3. What is geometrical isomerism in coordination compounds? Explain with examples.

Ans: Geometrical isomerism occurs due to different possible spatial arrangements of ligands around the central metal ion in coordination compounds, especially when coordination numbers are 4 or 6. For example, in square planar complexes like [Pt(NH₃)₂Cl₂], the two chloride ligands may be adjacent (cis isomer) or opposite each other (trans isomer). Similarly, octahedral complexes like [Co(NH₃)₄Cl₂]+ can have cis and trans forms. These isomers differ in physical and chemical properties.

4. Differentiate between homoleptic and heteroleptic complexes with examples.

Ans: Homoleptic complexes have only one type of ligand bonded to the metal ion, such as [Co(NH₃)₆]₃⁺, where all ligands are ammonia. Heteroleptic complexes have more than one kind of ligand, such as [Co(NH₃)₄Cl₂]⁺, which has ammonia and chloride ligands.

5. What is optical isomerism in coordination compounds? Describe its significance.

Ans: Optical isomerism occurs when coordination compounds exist as non-superimposable mirror images called enantiomers. These isomers rotate plane-polarized light in opposite directions (dextro and laevo forms). Optical isomerism is common in octahedral complexes with polydentate ligands like ethane-1,2-diamine ([Co(en)₃]₃₊). It is important in biological systems because enantiomers can have different biological activities.

6. What is crystal field splitting energy (Δo) and how does it affect the electronic configuration of octahedral complexes?

Ans: Crystal field splitting energy (Δo) is the energy difference between the higher energy eg orbitals and lower energy t₂g orbitals in an octahedral field due to ligand interactions. Depending on Δo relative to electron pairing energy (P), electrons may either pair up in lower orbitals (low spin complex) or occupy higher orbitals unpaired (high spin complex). Strong field ligands produce large Δo and low spin complexes, while weak field ligands produce small Δo and high spin complexes.

7. Describe the difference between a double salt and a coordination complex.

Ans: Double salts are formed by combining two salts in a definite stoichiometric ratio, and they dissociate completely into their constituent ions in solution. For example, Mohr’s salt dissociates fully into Fe²⁺, NH⁴⁺, and SO₄^2– ions. Coordination complexes contain a central metal ion bound to ligands forming a coordination sphere that does not dissociate completely. For example, [Fe(CN)₆]^4– remains intact in solution.

8. What is the role of coordination compounds in biological systems? Give examples.

Ans: Coordination compounds play crucial roles in biological processes. Chlorophyll, a magnesium coordination compound, is essential for photosynthesis. Haemoglobin, an iron coordination compound, carries oxygen in blood. Vitamin B12, a cobalt complex, is vital for red blood cell formation. Many enzymes also have coordinated metal ions that catalyze biochemical reactions.

9. Explain the synergic bonding in metal carbonyl complexes.

Ans: In metal carbonyls, bonding involves donation of a lone pair of electrons from the carbonyl carbon to the metal (σ-bond) and back-donation of electrons from filled metal d orbitals to empty antibonding π orbitals of CO (π-back bonding). This synergic bonding strengthens the metal–carbon bond and stabilizes the complex.