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આ MCQ મોડ્યુલ આના પર આધારિત છે: Werner Theory Nomenclature
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Werner Theory Nomenclature
🎓 Class 12
Chemistry
CBSE
Theory
Ch 5 – Coordination Compounds
⏱ ~14 min
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આ મૂલ્યાંકન આના પર આધારિત હશે: Werner Theory Nomenclature
મૂલ્યાંકન બનાવવામાં તેમની સામગ્રી સામેલ કરવા ચિત્રો, PDF અથવા Word દસ્તાવેજ અપલોડ કરો.
Werner Theory Nomenclature
5.1 Introduction — What is a Coordination Compound?
Transition metals are remarkable for forming a vast family of coordination compounds in which several anions or neutral molecules attach to one metal centre by sharing electron pairs. These compounds are essential to bio-inorganic chemistry (haemoglobin, chlorophyll, vitamin B12), to extractive metallurgy (Au, Ag, Ni purification), to catalysis (Wilkinson's catalyst), to medicine (cisplatin, EDTA chelation) and to electroplating.
A double salt vs. a complex. Carnallite KCl·MgCl2·6H2O and Mohr's salt FeSO4·(NH4)2SO4·6H2O are double salts — they completely dissociate into simple ions in water. A complex such as K4[Fe(CN)6] does not dissociate into Fe2+ and CN−; the [Fe(CN)6]4− ion remains intact.
5.2 Werner's Theory of Coordination Compounds
The Swiss chemist Alfred Werner (1866–1919), through painstaking conductivity, precipitation and isomerism experiments, proposed in 1893 that a metal ion shows two types of valence:
- Primary valence — ionisable, satisfied by negative ions; corresponds today to the oxidation state of the metal.
- Secondary valence — non-ionisable, satisfied by neutral molecules or anions held inside the coordination sphere; corresponds to the coordination number.
The classic experimental basis comes from the cobalt(III) chloride–ammonia series. When excess AgNO3 is added to fresh aqueous solutions of these compounds, only the chloride ions outside the coordination sphere precipitate as AgCl:
| Colour | Empirical formula | Modern formula | mol AgCl per mol | Conductivity |
|---|---|---|---|---|
| Yellow | CoCl3·6NH3 | [Co(NH3)6]3+ 3Cl− | 3 | 1 : 3 electrolyte |
| Purple | CoCl3·5NH3 | [CoCl(NH3)5]2+ 2Cl− | 2 | 1 : 2 electrolyte |
| Green | CoCl3·4NH3 | [CoCl2(NH3)4]+ Cl− | 1 | 1 : 1 electrolyte |
| Violet | CoCl3·4NH3 | [CoCl2(NH3)4]+ Cl− | 1 | 1 : 1 electrolyte |
The green and violet forms have the same formula but distinct properties — Werner called these isomers. He postulated that:
- Metals show two valences — primary (ionisable) and secondary (non-ionisable).
- Secondary valence equals the coordination number and is fixed for a given metal.
- The ions/groups bound by the secondary linkage occupy fixed positions in space — the coordination polyhedron.
- The most common polyhedra are octahedral, tetrahedral and square planar.
Worked Example 5.1 — Assigning Secondary Valence
From the moles of AgCl precipitated by excess AgNO3, assign the secondary valence (coordination number) for each compound below:
(i) PdCl2·4NH3 → 2 mol AgCl; (ii) NiCl2·6H2O → 2 mol AgCl; (iii) PtCl4·2HCl → 0 mol; (iv) CoCl3·4NH3 → 1 mol; (v) PtCl2·2NH3 → 0 mol AgCl.
The chlorides that precipitate are outside the coordination sphere; those that do not precipitate are inside. So the secondary valence = total ligands held inside.
(i) [Pd(NH3)4]Cl2 → CN = 4.
(ii) [Ni(H2O)6]Cl2 → CN = 6.
(iii) H2[PtCl6] → CN = 6.
(iv) [CoCl2(NH3)4]Cl → CN = 6.
(v) [PtCl2(NH3)2] → CN = 4.
(i) [Pd(NH3)4]Cl2 → CN = 4.
(ii) [Ni(H2O)6]Cl2 → CN = 6.
(iii) H2[PtCl6] → CN = 6.
(iv) [CoCl2(NH3)4]Cl → CN = 6.
(v) [PtCl2(NH3)2] → CN = 4.
5.3 Important Definitions
(a) Coordination entity, central atom and ligand
A coordination entity is a central metal atom/ion bonded to a fixed number of ions or molecules — for example [Co(NH3)6]3+. The metal centre acts as a Lewis acid; the surrounding species, called ligands, are Lewis bases.
(b) Denticity — uni-, di-, poly- and ambidentate
A ligand may use one or several donor atoms simultaneously:
| Type | Donor atoms used | Examples |
|---|---|---|
| Unidentate (monodentate) | 1 | Cl−, H2O, NH3, CN−, CO |
| Didentate (bidentate) | 2 | en (NH2CH2CH2NH2); ox (C2O42−) |
| Polydentate | ≥3 | EDTA4− (hexadentate, 2N + 4O); N(CH2CH2NH2)3 (tetradentate) |
| Ambidentate | 1 of 2 possible donor atoms | NO2− (N or O); SCN− (S or N) |
Chelate ligand & the chelate effect. When a di- or polydentate ligand grips a single metal ion through two or more donor atoms simultaneously, it forms a ring called a chelate. Chelated complexes are far more stable than complexes with the equivalent number of monodentate ligands — this is the chelate effect.
(c) Coordination number, sphere and polyhedron
The coordination number (CN) equals the number of σ-bonds the donor atoms make to the metal. Pi-bonds are not counted. Examples: in [PtCl6]2−, CN = 6. In [Fe(C2O4)3]3−, oxalate is didentate so CN = 6. In [Co(en)3]3+, en is didentate so CN = 6.
The metal and its ligands written together inside square brackets is the coordination sphere; ions written outside are counter ions. The 3-D shape of the donor atoms is the coordination polyhedron.
(d) Oxidation number of the central atom
The oxidation number is the charge the metal would carry if every ligand were removed along with the bonding electron pairs. It is shown by a Roman numeral in parentheses after the metal name, e.g. Cu(I) in [Cu(CN)4]3−.
(e) Homoleptic and heteroleptic complexes
Homoleptic = only one kind of donor atom (e.g. [Co(NH3)6]3+). Heteroleptic = more than one kind (e.g. [Co(NH3)4Cl2]+).
🧪 Activity 5.1 — The Disappearing Chloride (Predict → Observe → Explain)
Setup: Two beakers each contain 1 mol of cobalt-ammonia compound dissolved in water — beaker A has CoCl3·6NH3, beaker B has CoCl3·4NH3. Excess AgNO3(aq) is added to each.
Predict: How many moles of AgCl will precipitate from each beaker, and what does this tell you about how the chlorides are bound to the cobalt?
Observation: Beaker A → 3 mol AgCl. Beaker B → 1 mol AgCl.
Explanation: In A, all three Cl− are outside the coordination sphere ([Co(NH3)6]3+·3Cl−) and are free to react with Ag+. In B, two Cl− are bonded to cobalt as ligands ([CoCl2(NH3)4]+·Cl−), so they cannot be precipitated; only the single ionisable Cl− precipitates. This experiment is exactly how Werner deduced his theory.
5.4 Nomenclature of Coordination Compounds
The IUPAC scheme for naming a mononuclear complex uses additive nomenclature: ligands are listed as prefixes to the metal, with the metal's oxidation state shown in Roman numerals.
5.4.1 Writing the formula
- Central atom is listed first.
- Ligands follow in alphabetical order (charge does not matter).
- Polyatomic and abbreviated ligand formulas go in parentheses.
- The whole entity is enclosed in square brackets; the overall ion charge sits outside as a right superscript (e.g. [Cr(H2O)6]3+).
- Cation charges balance anion charges.
5.4.2 Writing the name
- Cation first, anion second (whether the complex part is cation or anion).
- Inside the complex, ligands are named in alphabetical order before the metal (the reverse of the formula rule for human readability).
- Anionic ligands end in -o/-ido (chlorido, cyanido, oxalato, hydroxido, nitrito-N, nitrito-O); neutral ligands keep their names except aqua (H2O), ammine (NH3), carbonyl (CO), nitrosyl (NO).
- Use di-, tri-, tetra- for simple ligands, but bis-, tris-, tetrakis- when the ligand name itself contains a numerical prefix (e.g. bis(ethane-1,2-diamine) for two en).
- The metal's oxidation state is given as a Roman numeral in parentheses.
- If the complex is an anion, the metal name ends in -ate (cobaltate, ferrate, cuprate, argentate, aurate, plumbate, stannate, zincate, nickelate).
| Ligand | Formula | Name in complex |
|---|---|---|
| Chloride | Cl− | chlorido |
| Bromide | Br− | bromido |
| Cyanide | CN− | cyanido |
| Hydroxide | OH− | hydroxido |
| Oxalate | C2O42− | oxalato |
| Nitrite (via N) | NO2− | nitrito-N |
| Nitrite (via O) | ONO− | nitrito-O |
| Water | H2O | aqua |
| Ammonia | NH3 | ammine |
| Carbon monoxide | CO | carbonyl |
| Ethane-1,2-diamine | en | ethane-1,2-diamine |
🔧 Interactive: Complex-Ion Namer & Oxidation-State Calculator
Pick a complex from the dropdown — the simulation derives the oxidation state of the metal, lists the ligands and constructs the IUPAC name step by step.
Pick a complex above.
Worked Example 5.2 — Naming from Formula
Write the IUPAC names: (a) [Pt(NH3)2Cl(NO2)], (b) K3[Cr(C2O4)3], (c) [CoCl2(en)2]Cl, (d) [Co(NH3)5(CO3)]Cl, (e) Hg[Co(SCN)4].
(a) diamminechloridonitrito-N-platinum(II). Charges: 2(0)+(−1)+(−1)+Pt = 0 → Pt = +2.
(b) potassium trioxalatochromate(III). 3(+1) + Cr + 3(−2) = 0 → Cr = +3.
(c) dichloridobis(ethane-1,2-diamine)cobalt(III) chloride. Inside complex: 2(−1)+0+Co = +1; outside Cl−; so Co = +3.
(d) pentaamminecarbonatocobalt(III) chloride. 5(0)+(−2)+Co = +1 (cation needs +1 because outside Cl−) → Co = +3.
(e) mercury(I) tetrathiocyanato-S-cobaltate(III).
(b) potassium trioxalatochromate(III). 3(+1) + Cr + 3(−2) = 0 → Cr = +3.
(c) dichloridobis(ethane-1,2-diamine)cobalt(III) chloride. Inside complex: 2(−1)+0+Co = +1; outside Cl−; so Co = +3.
(d) pentaamminecarbonatocobalt(III) chloride. 5(0)+(−2)+Co = +1 (cation needs +1 because outside Cl−) → Co = +3.
(e) mercury(I) tetrathiocyanato-S-cobaltate(III).
Worked Example 5.3 — Formula from Name
Write the formulas: (a) tetraammineaquachloridocobalt(III) chloride, (b) potassium tetrahydroxidozincate(II), (c) potassium trioxalatoaluminate(III), (d) dichloridobis(ethane-1,2-diamine)cobalt(III), (e) tetracarbonylnickel(0).
(a) [Co(NH3)4(H2O)Cl]Cl2
(b) K2[Zn(OH)4]
(c) K3[Al(C2O4)3]
(d) [CoCl2(en)2]+
(e) [Ni(CO)4]
(b) K2[Zn(OH)4]
(c) K3[Al(C2O4)3]
(d) [CoCl2(en)2]+
(e) [Ni(CO)4]
🎯 Competency-Based Questions
Q1. The coordination number of Co in [Co(en)3]3+ is: L1 Remember
Answer: (c) 6. Each en (ethane-1,2-diamine) is didentate, so 3 en × 2 donor atoms = 6.
Q2. Which of the following is an ambidentate ligand? L1 Remember
Answer: (c) NO2−. It can bond through N (nitrito-N) or through O (nitrito-O).
Q3. Find the oxidation state of Fe in K4[Fe(CN)6]. L3 Apply
4(+1) + Fe + 6(−1) = 0 → Fe + 4 − 6 = 0 → Fe = +2 (Fe(II)).
Q4. Compare a double salt (Mohr's salt) with a complex (potassium ferrocyanide) on dissolution behaviour, conductivity and ion availability. L4 Analyse
Answer: Mohr's salt FeSO4·(NH4)2SO4·6H2O dissociates fully into Fe2+, NH4+ and SO42− ions. So Fe2+ tests positive (e.g. with K3[Fe(CN)6] turbo blue colour). K4[Fe(CN)6] dissociates only into K+ and the intact [Fe(CN)6]4− ion; free Fe2+ is not detected. Conductivity matches the actual ions present.
Q5. Design a name for a hypothetical octahedral complex containing one carbonate, two ammonia, two chloride and one oxalato ligand bonded to chromium(III). L6 Create
Coordination number = 1 + 2 + 2 + 2(oxalato) = 7 — too many for octahedral, so we adjust to keep CN = 6: drop one Cl. Resulting entity [Cr(NH3)2Cl(C2O4)(CO3)]2−. Name (alphabetical): diamminechloridocarbonato(oxalato)chromate(III) ion. Note suffix -ate because complex is an anion (charge calculation: 2(0) + (−1) + (−2) + (−2) + Cr = −2 → Cr = +3 ✓).
🧠 Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: Addition of excess AgNO3 to [Co(NH3)5Cl]Cl2 gives 2 mol AgCl per mol of complex.
R: Only the chloride ions outside the coordination sphere are ionisable.
Answer: (A). Both true; R correctly explains A. The chloride bonded inside the sphere is non-ionisable; the two outside are free to precipitate as AgCl.
A: The chelate complex [Ni(en)3]2+ is more stable than [Ni(NH3)6]2+.
R: Polydentate ligands form ring structures around the metal.
Answer: (A). Both true; R explains A. The "chelate effect" — extra entropy gained when one polydentate ligand replaces several monodentate ones — increases stability.
A: In K3[Fe(CN)6] the metal name is written as ferrate(III).
R: When the complex ion is anionic, the metal name takes the suffix -ate.
Answer: (A). Both true; R is the rule that explains A. Hence "potassium hexacyanidoferrate(III)".
Frequently Asked Questions - Werner Theory Nomenclature
What is the main concept covered in Werner Theory Nomenclature?
In NCERT Class 12 Chemistry Chapter 5 (Coordination Compounds), "Werner Theory Nomenclature" covers the core chemistry principles and reactions students need for board exam success. The MyAiSchool lesson explains the topic with definitions, structural diagrams, reaction mechanisms, worked examples, and interactive simulations. Key reactions, IUPAC names, and chemical reasoning are highlighted throughout aligned with CBSE 2025-26 syllabus.
How is Werner Theory Nomenclature useful in real-life or applied chemistry?
Real-life applications of "Werner Theory Nomenclature" from NCERT Class 12 Chemistry Chapter 5 include drug design, polymer industry, food chemistry, electrochemical cells, fuel cells, dyes/pigments, agrochemicals, and biochemistry. The MyAiSchool lesson links every concept to a tangible industrial or biological example so students see chemistry as a problem-solving framework for the molecular world.
What are the key reactions students should memorize for Werner Theory Nomenclature?
Key reactions in "Werner Theory Nomenclature" (NCERT Class 12 Chemistry Chapter 5 Coordination Compounds) are tabulated in the MyAiSchool reaction map. Students should memorize each reaction with its reagent, conditions, mechanism class (SN1/SN2/E1/E2/electrophilic addition/etc), product, and stereochemistry. The Summary section provides a quick-reference reaction chart for last-minute revision.
How does this part connect to other parts of Chapter 5?
NCERT Class 12 Chemistry Chapter 5 (Coordination Compounds) is structured so each part builds chemical understanding sequentially. "Werner Theory Nomenclature" connects to neighbouring parts via shared functional groups, reaction mechanisms, and structural concepts. The MyAiSchool lesson cross-references related concepts with internal links so students can navigate the whole chapter as one connected story rather than disconnected fragments.
What types of CBSE board questions come from Werner Theory Nomenclature?
CBSE board questions from "Werner Theory Nomenclature" typically include: (1) 1-mark MCQs on definitions and IUPAC naming, (2) 2-mark short-answer reactions/products, (3) 3-mark mechanism questions, (4) 5-mark long-answer combining mechanism + product + stereochemistry + application. The MyAiSchool lesson tags each Competency-Based Question (CBQ) with Bloom level (L1-L6) so students know how to study for each weight.
How can students use the interactive simulation effectively?
The interactive simulation in the "Werner Theory Nomenclature" lesson allows students to explore reaction outcomes, predict products, or compare reaction conditions, with live visual feedback. To use it effectively: (1) try every option/configuration, (2) compare with the analytical reasoning, (3) check IUPAC names and structural correctness, (4) test edge cases from worked examples. The simulation reinforces conceptual intuition that pure mechanism memorisation cannot provide.
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