Exhaustive Guide: Werner's Theory & Nomenclature | Coordination Compounds | ChemCA

Coordination Compounds: Basics & IUPAC

Module 1 | CBSE Class 12 Chemistry | Coordination Compounds

1. Introduction to Coordination Compounds

Transition metals form a large number of complex compounds in which the metal atoms are bound to a number of anions or neutral molecules by sharing electrons. In modern terminology, such compounds are called Coordination Compounds.

These compounds play a vital role in biological systems (e.g., chlorophyll is a coordination compound of magnesium, hemoglobin is a coordination compound of iron, and vitamin B12 is a coordination compound of cobalt) and have extensive industrial and metallurgical applications.

2. Werner's Theory of Coordination Compounds

Alfred Werner (1893) was the first to formulate a theory to explain the structure and properties of coordination compounds. Based on his experiments, especially with cobalt amines like CoCl₃ · 6NH₃, he proposed his famous postulates:

Postulates of Werner's Theory:

In coordination compounds, metals show two types of linkages (valencies): primary and secondary.
Primary valencies are normally ionizable and are satisfied by negative ions. They correspond to the oxidation state of the metal.
Secondary valencies are non-ionizable. These are satisfied by neutral molecules or negative ions. They correspond to the coordination number of the metal.
The ions/groups bound by secondary linkages have characteristic spatial arrangements corresponding to different coordination numbers (e.g., octahedral, tetrahedral, square planar geometries).

Werner's Experiment Explained:
When aqueous AgNO₃ is added to CoCl₃ · 6NH₃, all three chloride ions are precipitated as AgCl. This means the 3 Cl^- are satisfying primary valencies (ionizable). The complex is written as [Co(NH₃)₆]Cl₃.
When AgNO₃ is added to CoCl₃ · 5NH₃, only two Cl^- precipitate. One Cl^- is satisfying both primary and secondary valency (non-ionizable). The complex is written as [Co(NH₃)₅Cl]Cl₂.

3. Important Terminology

1. Coordination Entity: Constitutes a central metal atom or ion bonded to a fixed number of ions or molecules. Example: [Co(NH₃)₃Cl₃] is a coordination entity.

2. Central Atom/Ion: The atom/ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement. It acts as a Lewis acid (electron pair acceptor).

3. Coordination Sphere and Counter Ions: The central atom/ion and the ligands attached to it are enclosed in a square bracket and is collectively termed as the coordination sphere. The ionizable groups written outside the bracket are called counter ions. Example: In K₄[Fe(CN)₆], [Fe(CN)₆]^4- is the coordination sphere and K⁺ is the counter ion.

4. Coordination Number (CN): The total number of ligand donor atoms to which the metal is directly bonded. For example, in [PtCl₆]^2-, the CN of Pt is 6.

3.1 Ligands and Denticity

Ligand: The ions or molecules bound to the central atom/ion in the coordination entity. They act as Lewis bases (electron pair donors).

Denticity: The number of coordinating or ligating atoms present in a ligand.

Classification	Description	NCERT Examples
Unidentate	Ligands with only one donor atom.	H₂O, NH₃, Cl^-, CN^-
Didentate	Ligands with two donor atoms.	Ethane-1,2-diamine (H₂N-CH₂-CH₂-NH₂, abbreviated as 'en'), Oxalate ion (C₂O₄^2-, 'ox')
Polydentate	Ligands with several donor atoms.	Hexadentate: EDTA^4- (ethylenediaminetetraacetate ion). It binds through two nitrogen and four oxygen atoms.

3.2 Chelate Effect & Ambidentate Ligands

These two concepts are heavily tested in 1-mark and assertion-reason questions.

Chelating Ligands & Chelate Effect: When a di- or polydentate ligand uses its two or more donor atoms to bind a single metal ion, it forms a ring structure. Such ligands are called chelating ligands. Complexes containing chelating rings are exceptionally stable. This extra stability is called the Chelate Effect.

Ambidentate Ligands: A unidentate ligand which can ligate (bind) through two different atoms present in it.
Examples:
1. Nitrite ion (NO₂^-): Can bind through Nitrogen (-NO₂) or through Oxygen (-ONO).
2. Thiocyanate ion (SCN^-): Can bind through Sulphur (-SCN) or through Nitrogen (-NCS).

4. IUPAC Nomenclature of Coordination Compounds

Naming coordination compounds systematically is a guaranteed 2-3 mark question in board exams. Follow these sequential rules strictly.

4.1 Rules for Naming

Order of naming ions: The cation is named first in both positively and negatively charged coordination entities. (Just like NaCl is sodium chloride).
Naming the Coordination Sphere: Ligands are named first in alphabetical order, followed by the name of the central metal atom/ion.
Names of Ligands:
- Anionic ligands end in -o (e.g., Cl^-: chlorido, CN^-: cyanido, C₂O₄^2-: oxalato).
- Neutral ligands keep their common names EXCEPT: H₂O (aqua), NH₃ (ammine - note the double 'm'), CO (carbonyl), and NO (nitrosyl).
Prefixes indicating number of ligands:
- Use di, tri, tetra, penta, hexa for simple ligands.
- Use bis, tris, tetrakis when the name of the ligand already includes a numerical prefix (like ethane-1,2-diamine). The ligand name is then placed in parentheses. Example: [Co(en)₃]³⁺ is tris(ethane-1,2-diamine)cobalt(III) ion.
Naming the Metal:
- If the complex ion is a cation or neutral, the metal name remains the same (e.g., Cobalt, Platinum).
- If the complex ion is an anion, the name of the metal ends with the suffix -ate. (e.g., Cobaltate, Platinate). For some metals, Latin names are used: Iron → Ferrate, Silver → Argentate, Copper → Cuprate, Gold → Aurate.
Oxidation State: The oxidation state of the metal in the complex is indicated by a Roman numeral in parentheses immediately following the metal name. (e.g., (II), (III), (IV)).

5. NCERT Solved Examples (Step-by-Step)

NCERT Example 9.2 (i): Write the IUPAC name for [Cr(NH₃)₃(H₂O)₃]Cl₃

Solution:
Step 1: Identify ions. Complex is the cation, Chloride (Cl^-) is the counter anion.
Step 2: Identify Ligands inside sphere. 3 Ammine (NH₃), 3 Aqua (H₂O). Alphabetically, 'ammine' comes before 'aqua'.
Step 3: Calculate Metal Oxidation State (O.S). Let Cr be x. x + 3(0) + 3(0) = +3 (charge on complex to balance 3Cl^-). So, x = +3.
Step 4: Metal Name. Complex is a cation, so metal is Chromium.
Full Name: Triamminetriaquachromium(III) chloride.

NCERT Example 9.2 (iv): Write the IUPAC name for K₃[Fe(CN)₆]

Solution:
Step 1: Identify ions. Potassium (K⁺) is the cation. The complex [Fe(CN)₆]^3- is the anion.
Step 2: Identify Ligands. 6 Cyanido (CN^-) ligands → hexacyanido.
Step 3: Calculate Metal O.S. Let Fe be x. x + 6(-1) = -3. So, x = +3.
Step 4: Metal Name. Since the complex is an anion, Iron becomes Ferrate.
Full Name: Potassium hexacyanidoferrate(III). (Note: We do not write 'tripotassium'. The counter ion number is implied by the oxidation states).

NCERT Example 9.3: Write the formula for Tetraammineaquachloridocobalt(III) chloride

Solution:
1. Central metal: Cobalt in +3 state (Co³⁺).
2. Ligands inside bracket: 4 ammine ((NH₃)₄), 1 aqua (H₂O), 1 chlorido (Cl).
3. Formula of coordination sphere: [Co(NH₃)₄(H₂O)Cl]ⁿ⁺.
4. Calculate charge on sphere (n+): (+3) + 4(0) + 1(0) + 1(-1) = +2. So sphere is [Co(NH₃)₄(H₂O)Cl]²⁺.
5. Counter anion: Chloride (Cl^-). To balance +2 charge, we need two Cl^-.
Final Formula: [Co(NH₃)₄(H₂O)Cl]Cl₂

6. Previous Year Questions (PYQs) & Exhaustive Question Bank

Part A: Conceptual (1-2 Marks)

[CBSE 2018, 2020]

Q1. What is meant by the chelate effect? Give an example.

Answer: When a di- or polydentate ligand binds to a central metal ion to form a ring-like structure, the resulting complex is exceptionally stable compared to similar complexes formed by unidentate ligands. This enhanced stability is called the chelate effect.
Example: [Cu(en)₂]²⁺ is much more stable than [Cu(NH₃)₄]²⁺ because 'en' (ethane-1,2-diamine) forms stable 5-membered rings with Cu²⁺.

[CBSE 2017, 2019]

Q2. Differentiate between ambidentate and didentate ligands.

Answer: An ambidentate ligand is a unidentate ligand that has two different donor atoms, but it binds to the metal using only one atom at a time (e.g., NO₂^- can bind via N or O).
A didentate ligand has two donor atoms and it binds to the metal using both atoms simultaneously, forming a ring (e.g., C₂O₄^2- uses two Oxygen atoms to bind).

Part B: Nomenclature (1 Mark each)

[CBSE 2016, 2021]

Q3. Write the IUPAC names of the following complexes:
(i) K₂[Zn(OH)₄]
(ii) [Pt(NH₃)₂Cl(NO₂)]

Answer:
(i) The complex is an anion. O.S of Zn = +2. Name: Potassium tetrahydroxidozincate(II).
(ii) The complex is neutral. O.S of Pt = +2. Ligands alphabetically: ammine, chlorido, nitrito-N. Name: Diamminechloridonitrito-N-platinum(II).

Part C: Werner's Theory Application (2 Marks)

[CBSE 2015, Sample Paper 2023]

Q4. When 1 mole of CoCl₃ · 5NH₃ is treated with excess of AgNO₃ solution, 2 moles of AgCl precipitate are formed. Formulate the complex and write its IUPAC name.

Answer:
Since 2 moles of AgCl precipitate, there are exactly two ionizable chloride ions present outside the coordination sphere acting as counter ions.
The remaining 1 Cl^- and 5 NH₃ molecules must be inside the coordination sphere to satisfy the secondary valency (coordination number of 6 for Cobalt).
Formula: [Co(NH₃)₅Cl]Cl₂
IUPAC Name: Pentaamminechloridocobalt(III) chloride.

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Exhaustive Guide: Werner's Theory & Nomenclature | Coordination Compounds