📝 Chapter Notes & Revision

Coordination Compounds

🏫 MP BoardClass 12Chemistry

📐 Formula & Cheat Sheet (English)

Class 12 Chemistry: Coordination Compounds - Quick Revision Notes

1. Introduction & Basic Terminology

Coordination compounds are addition molecular compounds that retain their identity even in solution. They contain a central metal atom or ion bonded to a fixed number of ions or molecules.

  • Central Metal Atom/Ion: The electron-deficient atom/ion (usually a transition metal) accepting electron pairs from ligands. Also called a Lewis acid.
  • Ligands: Ions or molecules bound to the central atom/ion through coordinate bonds. They act as Lewis bases.
  • Coordination Sphere: The central ion and the ligands attached to it are enclosed in square brackets [...], representing a single entity.
  • Counter Ions: The ionizable groups present outside the coordination sphere.
  • Coordination Number (CN): The total number of coordinate bonds formed by ligands with the central metal atom. (e.g., CN = 6 for octahedral, CN = 4 for tetrahedral/square planar).
  • Coordination Polyhedron: The spatial arrangement of the ligand atoms directly attached to the central metal (e.g., Octahedral, Tetrahedral, Square Planar).
  • Homoleptic Complexes: Complexes in which a metal is bound to only one kind of donor groups (e.g., [Co(NH3)6]3+).
  • Heteroleptic Complexes: Complexes in which a metal is bound to more than one kind of donor groups (e.g., [Co(NH3)4Cl2]+).

2. Classification of Ligands

Based on the number of donor sites:

  1. Unidentate: Ligands with one donor atom (e.g., Cl-, H2O, NH3, CN-).
  2. Didentate: Ligands with two donor atoms (e.g., Ethylene-diamine en: H2N-CH2-CH2-NH2, Oxalate ion ox: C2O4^2-).
  3. Polydentate: Ligands with multiple donor atoms (e.g., EDTA^4- is hexadentate).
  4. Chelate Ligands: Didentate or polydentate ligands that form a ring structure with the central metal, increasing stability (Chelate Effect).
  5. Ambidentate Ligands: Unidentate ligands that can ligate through two different atoms (e.g., -NO2 / -ONO, -SCN / -NCS).

3. IUPAC Nomenclature Rules

  1. Order of naming: Cation is named first in ionic complexes, followed by the anion.
  2. Naming Ligands: Ligands are named in alphabetical order before the metal.
    • Anionic ligands end in -o (e.g., Cl- = chloro/chlorido, SO4^2- = sulfato).
    • Neutral ligands have special names (H2O = aqua, NH3 = ammine, CO = carbonyl, NO = nitrosyl).
  3. Prefixes for multiplicity: If ligands are repeated, use di-, tri-, tetra-. For complex ligands, use bis-, tris-, tetrakis-.
  4. Oxidation State: The oxidation state of the central metal is written in Roman numerals in parentheses (II), (III) after its name.
  5. Ending of Metal Name:
    • If the complex ion is cationic or neutral, the metal name remains unchanged.
    • If the complex ion is anionic, the metal ends with suffix -ate (e.g., Iron becomes Ferrate, Cobalt becomes Cobaltate, Platinum becomes Platinate).

4. Isomerism in Coordination Compounds

Isomers are compounds that have the same chemical formula but different arrangements of atoms.

A. Stereoisomerism

  • Geometrical Isomerism: Found in heteroleptic complexes with CN = 4 or 6.
    • Cis-form: Similar groups lie adjacent to each other.
    • Trans-form: Similar groups lie opposite to each other.
    • Note: Tetrahedral complexes do not show geometrical isomerism. Square planar complexes of type [Ma2b2] show cis and trans.
  • Optical Isomerism: Exhibited by chiral complexes that are non-superimposable mirror images of each other. Common in octahedral complexes with didentate ligands (e.g., [Co(en)3]3+).

B. Structural Isomerism

  1. Linkage Isomerism: Shown by ambidentate ligands (e.g., [Co(NH3)5(NO2)]Cl and [Co(NH3)5(ONO)]Cl).
  2. Coordination Isomerism: Interchange of ligands between cationic and anionic entities of different metal ions.
  3. Ionisation Isomerism: Interchange of ions inside and outside the coordination sphere (e.g., [Co(NH3)5SO4]Br and [Co(NH3)5Br]SO4).
  4. Solvate (Hydrate) Isomerism: Water acts as a solvent inside or outside the coordination sphere (e.g., [Cr(H2O)6]Cl3 (violet) and [Cr(H2O)5Cl]Cl2.H2O (green-blue)).

5. Bonding Theories

A. Werner's Coordination Theory

  • Metals exhibit two types of valencies:
    1. Primary Valency: Ionisable, corresponds to oxidation state (satisfied by anions).
    2. Secondary Valency: Non-ionisable, corresponds to coordination number (satisfied by ligands; directed in specific spatial positions).

B. Valence Bond Theory (VBT)

  • Based on hybridisation of central metal ion to yield definite geometries.
  • CN = 4:
    • sp^3 hybridization $\rightarrow$ Tetrahedral (Paramagnetic/Diamagnetic depending on electrons).
    • dsp^2 hybridization $\rightarrow$ Square Planar (Diamagnetic usually for $d^8$).
  • CN = 6:
    • sp^3d^2 hybridization $\rightarrow$ Outer orbital complex (High spin / Spin-free).
    • d^2sp^3 hybridization $\rightarrow$ Inner orbital complex (Low spin / Spin-paired, caused by strong field ligands).
  • Limitations: Cannot explain the color of complexes or detailed magnetic data quantitatively.

C. Crystal Field Theory (CFT)

  • Assumes ligands are point charges and the metal-ligand bond is purely electrostatic.
  • Splitting in Octahedral Complexes ($\Delta_o$):
    • The degenerate $d$-orbitals split into two sets: lower energy $t_{2g}$ set ($d_{xy}, d_{yz}, d_{zx}$) and higher energy $e_g$ set ($d_{x^2-y^2}, d_{z^2}$).
    • Energy gap = $\Delta_o$.
    • If pairing energy ($P$) > $\Delta_o$, electrons go to $e_g$ (High Spin Complex, weak field ligands like $\text{I}^- < \text{Br}^- < \text{Cl}^-$).
    • If pairing energy ($P$) < $\Delta_o$, electrons pair up in $t_{2g}$ (Low Spin Complex, strong field ligands like $\text{CN}^- > \text{CO} > \text{en} > \text{NH}_3$).
  • Spectrochemical Series (Increasing field strength): I- < Br- < SCN- < Cl- < S2- < F- < OH- < C2O4^2- < H2O < NCS- < edta^4- < NH3 < en < NO2- < CN- < CO
  • Splitting in Tetrahedral Complexes ($\Delta_t$):
    • $\Delta_t = \frac{4}{9} \Delta_o$.
    • The $e_g$ set is lower in energy, and the $t_{2g}$ set is higher. Tetrahedral complexes are always high spin.

6. Color and Magnetic Properties

  • Color: Arises due to d-d electron transition in the visible region. An electron absorbs a specific wavelength of light to jump from $t_{2g}$ to $e_g$ level.
  • Magnetic Moment ($\mu$): Calculated using the spin-only formula: $$\mu = \sqrt{n(n + 2)}$$ (where $n$ = number of unpaired electrons, unit is Bohr Magneton, BM).

7. Important Applications

  1. Extraction of Metals: Extraction of gold and silver via cyanide process (Leaching).
  2. Medicine:
    • EDTA: Used for treatment of lead poisoning.
    • Cisplatin: Used as an anti-cancer drug.
    • Vitamin B12: A coordination compound of Cobalt.
  3. Biological Systems: Chlorophyll is a coordination compound of Magnesium; Hemoglobin is of Iron.
  4. Analytical Chemistry: Estimation of hardness of water using EDTA.