Chapter 5: Coordination Compounds Case Study Questions | chemistry Class 12 CBSE

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Chapter 5: Coordination Compounds Case Study Questions | chemistry Class 12 CBSE | ByteNotes.in

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Alfred Werner, a Swiss chemist, studied a series of cobalt(III) chloride-ammonia complexes. He observed that when excess silver nitrate solution was added to CoCl3·6NH3 (yellow), CoCl3·5NH3 (purple), CoCl3·4NH3 (green), and CoCl3·4NH3 (violet) in cold aqueous solution, different amounts of AgCl precipitate were obtained. The yellow compound gave 3 mol AgCl, the purple gave 2 mol AgCl, and both the green and violet compounds gave only 1 mol AgCl per mole of compound. Conductivity measurements confirmed these observations. Werner proposed that the cobalt ion uses two types of valences — primary and secondary — to explain these results, and formulated the modern concept of coordination sphere using square brackets.

Question 1: Write the modern formula for the yellow complex CoCl3·6NH3 and identify its counter ions.

The modern formula is [Co(NH3)6]3+ 3Cl–
The counter ions are three Cl– ions present outside the coordination sphere (square brackets).

Question 2: Why do CoCl3·4NH3 (green) and CoCl3·4NH3 (violet) give the same number of moles of AgCl yet have distinct properties?

Both have the same empirical formula CoCl3·4NH3 and formula [CoCl2(NH3)4]+Cl–, giving only 1 mol AgCl.
They are isomers — compounds with the same chemical formula but different arrangement of atoms (spatial arrangement of ligands differs), leading to distinct physical and chemical properties.

Question 3: State the four main postulates of Werner's theory of coordination compounds and explain how they account for the observations described in the passage.

(1) Metals show two types of valences: primary (ionisable, satisfied by negative ions) and secondary (non-ionisable, satisfied by neutral molecules or negative ions).
(2) Primary valence corresponds to the oxidation state of the metal; secondary valence equals the coordination number and is fixed for a metal.
(3) The ions/groups bound by secondary linkages have characteristic spatial (geometrical) arrangements — coordination polyhedra.
In the passage, all four cobalt complexes have Co in +3 oxidation state (primary valence = 3); secondary valence = 6 in each case. Only the Cl– ions outside the square bracket (primary valence) are precipitated by AgNO3, explaining the different moles of AgCl obtained.

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Case Set 2

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A student in a laboratory is studying various ligands and their classification. She observes that ethylenediaminetetraacetate ion (EDTA4–) can bind to a metal ion through two nitrogen atoms and four oxygen atoms simultaneously. She also notices that ethane-1,2-diamine (en) binds through two nitrogen donor atoms, while NH3 binds through only one nitrogen atom. In another experiment, she finds that the NO2– ion can coordinate either through nitrogen (giving a nitro complex) or through oxygen (giving a nitrito complex). The student understands that polydentate ligands that use multiple donor atoms simultaneously to bind a single metal ion form chelate complexes, which are generally more stable than complexes with unidentate ligands.

Question 1: Classify EDTA4– and NH3 based on their denticity and give their coordination mode.

EDTA4– is a hexadentate ligand — it binds through 2 nitrogen and 4 oxygen atoms simultaneously to a central metal ion.
NH3 is a unidentate ligand — it binds through only one donor atom (nitrogen).

Question 2: What is an ambidentate ligand? Identify the ambidentate ligand from the passage and explain its two modes of coordination.

An ambidentate ligand has two different donor atoms and can coordinate through either one in a complex.
NO2– is the ambidentate ligand in the passage. It can coordinate through nitrogen (–NO2, nitro mode) or through oxygen (–ONO, nitrito mode) to the central metal atom/ion.

Question 3: Define chelate ligand and chelate complex. Explain why chelate complexes are more stable than analogous complexes with unidentate ligands (chelate effect). Use en as an example.

A chelate ligand is a di- or polydentate ligand that uses two or more donor atoms simultaneously to bind a single metal ion, forming a ring structure.
A chelate complex is the resulting complex formed by such chelate ligands.
Chelate complexes are more stable due to the chelate effect: when a polydentate ligand binds, replacing multiple unidentate ligands increases the number of free particles in solution (entropy increases), making the process thermodynamically more favourable.
Example: [Co(en)3]3+ (en = ethane-1,2-diamine, a didentate chelate ligand) is more stable than [Co(NH3)6]3+ because en forms two bonds per molecule, and its displacement releases fewer species.

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Case Set 3

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A chemistry teacher demonstrates IUPAC nomenclature of coordination compounds to her class. She writes the formula [Cr(NH3)3(H2O)3]Cl3 on the board and explains that the cation is named first, ligands are listed alphabetically before the metal, and the oxidation state of the metal is given in Roman numerals in parentheses. She also explains that anionic ligands end in '–o', while neutral ligands retain their names except for special cases like aqua (H2O), ammine (NH3), carbonyl (CO), and nitrosyl (NO). When the same ligand appears multiple times, prefixes mono, di, tri are used, or bis, tris, tetrakis when the ligand name already contains a numerical prefix.

Question 1: Name the compound [Cr(NH3)3(H2O)3]Cl3 using IUPAC rules and determine the oxidation number of chromium.

IUPAC name: triamminetriaquachromium(III) chloride.
All ligands (NH3 and H2O) are neutral, so oxidation number of Cr = charge on complex ion = +3.

Question 2: Write the IUPAC name of [Co(H2NCH2CH2NH2)3]2(SO4)3 and explain why the prefix 'tris' is used instead of 'tri'.

IUPAC name: tris(ethane-1,2-diamine)cobalt(III) sulphate.
The prefix 'tris' is used (instead of 'tri') because the name of the ligand, ethane-1,2-diamine, already contains a numerical component, so to avoid ambiguity, the terms bis, tris, tetrakis are used with the ligand name enclosed in parentheses.

Question 3: Write the formulas for the following: (a) potassium trioxalatoaluminate(III), (b) tetraammineaquachloridocobalt(III) chloride, (c) dichloridobis(ethane-1,2-diamine)cobalt(III). State the rules used for writing formulas of coordination entities.

(a) K3[Al(C2O4)3]; (b) [Co(NH3)4(H2O)Cl]Cl2; (c) [CoCl2(en)2]+.
Rules: (i) Central atom is listed first; (ii) ligands are listed alphabetically; (iii) the entire coordination entity is enclosed in square brackets; (iv) polyatomic ligand formulas are in parentheses; (v) no space between ligands and metal within coordination sphere; (vi) charge of cation is balanced by charge of anion.

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Geometrical isomerism is an important type of stereoisomerism in coordination compounds. A student studies two platinum complexes: cis-[Pt(NH3)2Cl2] and trans-[Pt(NH3)2Cl2]. In the cis isomer, the two Cl ligands are adjacent (on the same side), while in the trans isomer, they are opposite to each other. The student extends this study to octahedral complexes and finds that [Co(NH3)4Cl2]+ exists as cis and trans isomers. She also notes that for the type [Ma3b3] in octahedral geometry, facial (fac) and meridional (mer) isomers are possible, as seen in [Co(NH3)3(NO2)3]. Tetrahedral complexes, however, do not exhibit geometrical isomerism.

Question 1: Define geometrical isomerism and state why it does not occur in tetrahedral complexes with two different unidentate ligands.

Geometrical isomerism arises in heteroleptic complexes due to different possible spatial arrangements of ligands around the central metal ion (cis and trans positions).
Tetrahedral complexes do not show geometrical isomerism because the relative positions of unidentate ligands attached to the central metal atom are equivalent with respect to each other — all positions are adjacent.

Question 2: Differentiate between fac and mer isomers in octahedral complexes of the type [Ma3b3], with reference to [Co(NH3)3(NO2)3].

In the facial (fac) isomer of [Co(NH3)3(NO2)3], three identical donor atoms (e.g., three NH3) occupy adjacent positions at the corners of one octahedral face.
In the meridional (mer) isomer, the three identical donor atoms are arranged around the meridian (equatorial plane) of the octahedron — one trans pair and one in a different position.

Question 3: How many geometrical isomers are possible for [Co(NH3)3Cl3]? Draw their structures and name them. Also explain whether they can show optical isomerism.

Two geometrical isomers are possible: fac-[Co(NH3)3Cl3] and mer-[Co(NH3)3Cl3].
In the fac isomer, all three NH3 (or all three Cl) are on one face of the octahedron; in the mer isomer, they are arranged around the meridian.
The fac isomer has a C3 axis and planes of symmetry; it is superimposable on its mirror image, so it is NOT optically active. The mer isomer also has a plane of symmetry and is not optically active. Neither isomer shows optical isomerism.

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Case Set 5

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Optical isomerism is a type of stereoisomerism where mirror image structures (enantiomers) are non-superimposable. A student investigates [Co(en)3]3+ and finds that it exists as dextro (d) and laevo (l) forms, rotating plane-polarised light to the right and left respectively. These molecules are termed chiral. The student then examines cis-[PtCl2(en)2]2+ and confirms it shows optical activity, but the trans isomer does not. She also studies cis-[CrCl2(ox)2]3– and trans-[CrCl2(ox)2]3– and concludes that only the cis form is chiral. Optical isomerism is common in octahedral complexes involving didentate ligands.

Question 1: Define enantiomers and state what property makes a molecule or ion chiral.

Enantiomers are mirror images of each other that cannot be superimposed on one another.
A molecule or ion is chiral when it lacks a plane of symmetry (or centre of symmetry), making it non-superimposable on its mirror image.

Question 2: Why does the trans isomer of [PtCl2(en)2]2+ not show optical activity, while the cis isomer does?

The trans-[PtCl2(en)2]2+ has a plane of symmetry passing through the molecule, making it superimposable on its mirror image — hence it is achiral and optically inactive.
The cis-[PtCl2(en)2]2+ lacks any plane of symmetry, so its mirror image is non-superimposable on itself — it is chiral and optically active.

Question 3: Explain with structural reasoning why [Co(en)3]3+ is optically active. How many stereoisomers does this complex have in total? Name and describe each.

[Co(en)3]3+ has three bidentate en ligands arranged around Co3+ in an octahedral geometry. The resulting propeller-like arrangement has no plane or centre of symmetry.
Its mirror image is non-superimposable, making it chiral and hence optically active.
Total stereoisomers: 2 — the d (dextro) form, which rotates plane-polarised light to the right, and the l (laevo) form, which rotates it to the left. Both are optical isomers (enantiomers) of each other with identical physical properties except for the direction of rotation of plane-polarised light.

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Chapter 5: Coordination Compounds | Case Study Questions

Passage

Question 1: Write the modern formula for the yellow complex CoCl3·6NH3 and identify its counter ions.

Question 2: Why do CoCl3·4NH3 (green) and CoCl3·4NH3 (violet) give the same number of moles of AgCl yet have distinct properties?

Question 3: State the four main postulates of Werner's theory of coordination compounds and explain how they account for the observations described in the passage.

Passage

Question 1: Classify EDTA4– and NH3 based on their denticity and give their coordination mode.

Question 2: What is an ambidentate ligand? Identify the ambidentate ligand from the passage and explain its two modes of coordination.

Question 3: Define chelate ligand and chelate complex. Explain why chelate complexes are more stable than analogous complexes with unidentate ligands (chelate effect). Use en as an example.

Passage

Question 1: Name the compound [Cr(NH3)3(H2O)3]Cl3 using IUPAC rules and determine the oxidation number of chromium.

Question 2: Write the IUPAC name of [Co(H2NCH2CH2NH2)3]2(SO4)3 and explain why the prefix 'tris' is used instead of 'tri'.

Question 3: Write the formulas for the following: (a) potassium trioxalatoaluminate(III), (b) tetraammineaquachloridocobalt(III) chloride, (c) dichloridobis(ethane-1,2-diamine)cobalt(III). State the rules used for writing formulas of coordination entities.

Passage

Question 1: Define geometrical isomerism and state why it does not occur in tetrahedral complexes with two different unidentate ligands.

Question 2: Differentiate between fac and mer isomers in octahedral complexes of the type [Ma3b3], with reference to [Co(NH3)3(NO2)3].

Question 3: How many geometrical isomers are possible for [Co(NH3)3Cl3]? Draw their structures and name them. Also explain whether they can show optical isomerism.

Passage

Question 1: Define enantiomers and state what property makes a molecule or ion chiral.

Question 2: Why does the trans isomer of [PtCl2(en)2]2+ not show optical activity, while the cis isomer does?

Question 3: Explain with structural reasoning why [Co(en)3]3+ is optically active. How many stereoisomers does this complex have in total? Name and describe each.

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