Coordination Compounds - Class 12 Chemistry | Chemca.in

Module 9 • Maharashtra HSC Board

Coordination Compounds

Comprehensive Theory, VBT, IUPAC Nomenclature & Fully Solved Board PYQs

1. Introduction and Basic Terminology

When solutions of two or more stable simple salts are mixed together and evaporated, new substances called addition compounds are formed. They are of two types:

Double Salts	Coordination (Complex) Compounds
Completely dissociate into simple ions when dissolved in water.	Do not completely dissociate into simple ions in water. The complex ion remains intact.
Lose their identity in aqueous solution.	Retain their identity in solid as well as in aqueous solution.
Give tests for all their constituent ions.	Do not give tests for the ions present inside the coordination sphere.
Example: Mohr's salt $FeSO_4\cdot(NH_4)_2SO_4\cdot6H_2O$	Example: Potassium hexacyanoferrate(II) $K_4[Fe(CN)_6]$

Basic Terms

Coordination Sphere: The central metal atom/ion and the ligands attached to it are enclosed in a square bracket called the coordination sphere. The ionizable groups outside the bracket are called counter ions.
Ligands: The molecules or ions that are coordinated (bonded) to the central metal atom or ion by donating an electron pair. They act as Lewis bases.
- Monodentate: Donate one electron pair (e.g., $Cl^-, H_2O, NH_3$).
- Bidentate: Donate two electron pairs (e.g., ethylenediamine ($en$), oxalate ion ($C_2O_4^{2-}$)).
- Ambidentate: A monodentate ligand that can coordinate through two different atoms. (e.g., $-NO_2$ can bind through N or O; $-SCN$ can bind through S or N).
Coordination Number (C.N.): The total number of coordinate bonds formed by ligands with the central metal atom/ion. (e.g., in $[Cu(NH_3)_4]^{2+}$, C.N. = 4).

2. Werner's Theory of Coordination Compounds

Alfred Werner (the father of coordination chemistry) proposed this theory in 1898 to explain the structure of complex compounds.

Postulates of Werner's Theory:

In coordination compounds, most metals exhibit two types of valency:
- Primary Valency: It is ionizable, non-directional, and corresponds to the oxidation state of the central metal ion. It is satisfied by negative ions.
- Secondary Valency: It is non-ionizable, directional, and corresponds to the coordination number of the central metal. It is satisfied by negative ions or neutral molecules (ligands).
Every metal atom has a fixed number of secondary valencies (fixed coordination number).
The secondary valencies are directed towards fixed positions in space, which gives a definite geometry to the coordination compound (e.g., octahedral for C.N. 6, tetrahedral or square planar for C.N. 4).

3. Effective Atomic Number (EAN) Rule

Sidgwick proposed the EAN rule. According to this rule, the central metal ion in a complex accepts electron pairs from ligands until the total number of electrons around it becomes equal to the atomic number of the nearest noble gas.

$$ \text{EAN} = Z - X + Y $$

$Z$ = Atomic number of the central metal atom.
$X$ = Number of electrons lost (Oxidation state).
$Y$ = Number of electrons donated by ligands ($2 \times \text{Coordination Number}$).

Example: Calculate EAN of $Fe$ in $[Fe(CN)_6]^{4-}$

Atomic number of Fe ($Z$) = 26.
Oxidation state of Fe in the complex ($X$) = +2.
Electrons donated by 6 $CN^-$ ligands ($Y$) = $6 \times 2 = 12$.
$\text{EAN} = 26 - 2 + 12 = 36$. (This equals the atomic number of Krypton, hence EAN rule is obeyed).

4. IUPAC Nomenclature of Coordination Compounds

The rules for naming complex compounds are:

The cation is named first, followed by the anion, regardless of whether the complex ion is the cation or the anion.
Naming the coordination sphere: Ligands are named first in alphabetical order, followed by the name of the central metal.
Prefixes like di, tri, tetra, penta, hexa are used to indicate the number of identical ligands. (If the ligand name itself contains a prefix, use bis, tris, tetrakis).
Names of Ligands:
- Anionic ligands end in '-o' (e.g., $Cl^-$: chloro/chlorido, $CN^-$: cyano).
- Neutral ligands have specific names (e.g., $H_2O$: aqua, $NH_3$: ammine, $CO$: carbonyl).
Name of the Metal:
- If the complex ion is a cation or neutral, the metal name is the same as the element.
- If the complex ion is an anion, the name of the metal ends with the suffix '-ate'. (e.g., Ferrate for Fe, Cuprate for Cu, Argentate for Ag).
The oxidation state of the metal is written in Roman numerals in parentheses immediately after the metal name.

Examples of IUPAC Nomenclature:

$K_4[Fe(CN)_6]$: Potassium hexacyanoferrate(II)
$[Co(NH_3)_5Cl]Cl_2$: Pentaamminechlorocobalt(III) chloride
$[Ni(CO)_4]$: Tetracarbonylnickel(0)
$K_2[PtCl_6]$: Potassium hexachloroplatinate(IV)

5. Isomerism in Coordination Compounds

Isomers are compounds having the same molecular formula but different structural arrangements.

A. Structural Isomerism

Ionization Isomerism: Isomers that give different ions in solution.
Example: $[Co(NH_3)_5Br]SO_4$ (gives $SO_4^{2-}$ test) and $[Co(NH_3)_5SO_4]Br$ (gives $Br^-$ test).
Linkage Isomerism: Occurs when an ambidentate ligand is present.
Example: $[Co(NH_3)_5(NO_2)]Cl_2$ (yellow, bonded through N) and $[Co(NH_3)_5(ONO)]Cl_2$ (red, bonded through O).
Coordination Isomerism: Arises from the interchange of ligands between cationic and anionic entities of different metals present in the complex.
Example: $[Cu(NH_3)_4][PtCl_4]$ and $[Pt(NH_3)_4][CuCl_4]$.
Hydrate (Solvate) Isomerism: Differs in the number of solvent (water) molecules directly bonded to the metal ion versus those in the crystal lattice.
Example: $[Cr(H_2O)_6]Cl_3$ (Violet) and $[Cr(H_2O)_5Cl]Cl_2\cdot H_2O$ (Light green).

B. Stereoisomerism

Geometrical Isomerism: Common in square planar ($MA_2B_2$) and octahedral complexes ($MA_4B_2$).
- Cis-isomer: Identical ligands occupy adjacent positions (angle $90^\circ$).
- Trans-isomer: Identical ligands occupy opposite positions (angle $180^\circ$).
Optical Isomerism: Isomers that are non-superimposable mirror images of each other. Common in octahedral complexes involving bidentate ligands (e.g., $[Co(en)_3]^{3+}$). They exist as dextro (d) and laevo (l) forms.

6. Valence Bond Theory (VBT)

Proposed by Linus Pauling. It explains the structure and magnetic properties of coordination compounds based on hybridization.

Postulates of VBT:

The central metal atom provides empty orbitals for the formation of coordinate bonds with ligands.
These empty orbitals hybridize to form a new set of equivalent orbitals with definite geometry. (e.g., $sp^3$ = tetrahedral, $dsp^2$ = square planar, $sp^3d^2$ or $d^2sp^3$ = octahedral).
Ligand Strength:
- Strong Field Ligands: ($CN^-, CO, en, NH_3$) force the pairing of unpaired d-electrons of the metal ion against Hund's rule.
- Weak Field Ligands: ($Cl^-, F^-, H_2O$) do not cause pairing of electrons.
If inner $(n-1)d$ orbitals are used for hybridization ($d^2sp^3$), it forms an Inner Orbital Complex (usually low spin). If outer $nd$ orbitals are used ($sp^3d^2$), it forms an Outer Orbital Complex (usually high spin).

7. Crystal Field Theory (CFT) - Brief Overview

CFT considers the metal-ligand bond to be purely ionic. When ligands approach the central metal ion, the five degenerate d-orbitals split into two sets of different energies.

Octahedral Splitting: The d-orbitals split into a lower energy $t_{2g}$ set ($d_{xy}, d_{yz}, d_{zx}$) and a higher energy $e_g$ set ($d_{x^2-y^2}, d_{z^2}$). The energy difference is called Crystal Field Splitting Energy ($\Delta_o$).

If $\Delta_o > P$ (Pairing energy), strong field ligand $\rightarrow$ electrons pair up in $t_{2g}$ $\rightarrow$ Low spin complex.
If $\Delta_o < P$, weak field ligand $\rightarrow$ electrons enter $e_g$ before pairing $\rightarrow$ High spin complex.

8. Solved Textbook Numericals & Reasoning

Problem 1: Application of VBT on $[Ni(CN)_4]^{2-}$

Explain the geometry and magnetic property of $[Ni(CN)_4]^{2-}$ on the basis of VBT. (Atomic number of Ni = 28).

Solution:

1. Oxidation state of Ni in $[Ni(CN)_4]^{2-}$: $x + 4(-1) = -2 \implies x = +2$.

2. Electronic configuration of Ni ($Z=28$): $[Ar] 3d^8 4s^2$.

3. Electronic configuration of $Ni^{2+}$: $[Ar] 3d^8$.

In $3d$ subshell, there are 3 pairs and 2 unpaired electrons.

4. $CN^-$ is a strong field ligand. It forces the pairing of the two unpaired 3d electrons. This leaves one 3d orbital empty.

5. Hybridization: The empty $3d$, $4s$, and two $4p$ orbitals hybridize to give four $dsp^2$ hybrid orbitals.

6. Geometry: $dsp^2$ hybridization corresponds to a Square Planar geometry.

7. Magnetic Property: Since all electrons are paired after the approach of $CN^-$, the complex is diamagnetic ($\mu = 0$).

Problem 2: Effective Atomic Number (EAN)

Calculate the EAN of Cobalt in $[Co(NH_3)_6]^{3+}$. (Atomic number of Co = 27).

Solution:

Atomic Number of Co ($Z$) = 27

Oxidation state of Co ($X$): Let it be $x$. $x + 6(0) = +3 \implies x = 3$. Electrons lost = 3.

Coordination Number = 6. Electrons donated by ligands ($Y$) = $6 \times 2 = 12$.

Formula: $\text{EAN} = Z - X + Y$

$\text{EAN} = 27 - 3 + 12 = 36$

Answer: The EAN is 36, which obeys the EAN rule (Atomic number of Kr).

9. Board PYQs with Complete Answers

Verified previous year questions from the Maharashtra State Board HSC Chemistry exams.

1 Mark Questions (VSA)

Q1. Define: Ambidentate ligand. (March 2013, Oct 2017)

Answer: A monodentate ligand which contains more than one donor atom and can coordinate to the central metal ion through either of the two atoms is called an ambidentate ligand. (Example: $-NO_2$ can bind through N or O).

Q2. Write the IUPAC name of $[Co(NH_3)_5(H_2O)]Cl_3$. (March 2018, March 2022)

Answer: Pentaammineaquacobalt(III) chloride.

Q3. What is the coordination number of Pt in $[Pt(en)_2Cl_2]^{2+}$? (March 2016, Oct 2020)

Answer: Ethylenediamine (en) is a bidentate ligand. Number of bonds = $2 \times 2 (\text{from en}) + 2 (\text{from Cl}) = 6$. The coordination number is 6.

2 Mark Questions (SA-I)

Q4. State any two postulates of Werner's theory of coordination compounds. (March 2014, Oct 2019, March 2023)

Answer:

Most elements exhibit two types of valency: Primary valency (ionizable, satisfies oxidation state) and Secondary valency (non-ionizable, satisfies coordination number).
The secondary valencies are directed towards fixed positions in space, giving a definite geometry to the complex.

Q5. Distinguish between a double salt and a coordination compound. (March 2015, March 2021)

Answer:

Double Salt: Dissociates completely into simple constituent ions in aqueous solution and loses its identity. Example: Mohr's salt.
Coordination Compound: Does not dissociate completely in water; the complex ion remains intact, retaining its identity. Example: $K_4[Fe(CN)_6]$.

3 Mark Questions (SA-II)

Q6. Explain the geometry, hybridization, and magnetic property of $[CoF_6]^{3-}$ on the basis of Valence Bond Theory (VBT). (Z for Co = 27). (Oct 2014, March 2019)

Answer:

1. Oxidation state of Co is +3. Configuration of Co ($Z=27$) is $[Ar] 3d^7 4s^2$. Configuration of $Co^{3+}$ is $[Ar] 3d^6$.

2. In the 3d subshell, there is 1 paired and 4 unpaired electrons.

3. Fluoride ($F^-$) is a weak field ligand. It cannot force the pairing of 3d electrons against Hund's rule.

4. Hybridization: Since 3d orbitals are unavailable, the complex uses outer orbitals: one $4s$, three $4p$, and two $4d$ orbitals to undergo $sp^3d^2$ hybridization.

5. Geometry: $sp^3d^2$ hybridization gives an Octahedral geometry.

6. Magnetic Property: Since there are 4 unpaired electrons present in the 3d subshell, the complex is highly paramagnetic (Outer orbital/high spin complex).

Q7. (A) State Effective Atomic Number (EAN) rule. (B) Calculate EAN of central metal in $[Fe(CO)_5]$. (Z for Fe = 26). (March 2017, Oct 2021)

Answer:

Part A: EAN Rule

According to the EAN rule, a central metal ion forms coordinate bonds with ligands until the total number of electrons surrounding it becomes equal to the atomic number of the nearest noble gas.

Part B: EAN Calculation for $[Fe(CO)_5]$

1. Atomic number of Fe ($Z$) = 26.

2. Oxidation state of Fe ($X$): Since CO is a neutral ligand, $x + 5(0) = 0 \implies x = 0$. Electrons lost = 0.

3. Number of electrons donated by 5 CO ligands ($Y$) = $5 \times 2 = 10$.

4. $\text{EAN} = Z - X + Y$

$\text{EAN} = 26 - 0 + 10 = 36$

Final Answer: The EAN is 36, which is the atomic number of Krypton. Rule is obeyed.

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