Coordination Compounds

Coordination Chemistry: The Metal’s Best Friends

The Big Idea (One Analogy to Rule Them All)

Imagine a KING sitting on a throne surrounded by loyal servants. The king is a metal ion (like Fe²⁺ or Cu²⁺), and the servants are ligands (molecules or ions that donate electrons). Together, they form a coordination compound — a royal court where the king and servants work as one team!

1. Coordination Compound Basics

What IS a Coordination Compound?

Think of it like this: A metal atom is lonely. It wants friends. So, molecules or ions come and donate their electrons to the metal, forming bonds. This whole “metal + friends” group is called a coordination compound.

         Ligand (friend)
              ↓
    [Metal ← Ligand]  = Coordination Compound
              ↑
         Donates electrons

The Building Blocks

Example: In [Cu(NH₃)₄]²⁺:

• Cu²⁺ = the metal king
• NH₃ = four ammonia servants
• The brackets show the coordination sphere

2. Coordination Number

What’s a Coordination Number?

It’s simply how many servants (ligands) are directly connected to the king (metal).

Think of it as: How many hands is the king holding?

Common Coordination Numbers

graph TD
    A["Coordination Numbers"] --> B["CN = 2"]
    A --> C["CN = 4"]
    A --> D["CN = 6"]
    B --> B1["Linear<br>[Ag#40;NH₃#41;₂]⁺"]
    C --> C1["Square Planar<br>[PtCl₄]²⁻"]
    C --> C2["Tetrahedral<br>[ZnCl₄]²⁻"]
    D --> D1["Octahedral<br>[Fe#40;CN#41;₆]⁴⁻"]

Key Point: The coordination number depends on:

• Size of the metal
• Size of the ligands
• Electronic configuration

3. Ligand Types

What Makes a Good Servant?

Ligands have lone pairs of electrons they can share with the metal. Based on HOW MANY electron pairs they donate, we classify them:

Classification by Donation Sites

graph TD
    L["Ligand Types"] --> M["Monodentate"]
    L --> B["Bidentate"]
    L --> P["Polydentate"]
    M --> M1["ONE donation site<br>Like a servant with 1 hand"]
    B --> B1["TWO donation sites<br>Like a servant with 2 hands"]
    P --> P1["MANY donation sites<br>Like an octopus servant!"]

Special Ligand Categories

Ambidentate Ligands: Can bond through different atoms!

• NO₂⁻ → bonds via N (nitro) OR O (nitrito)
• SCN⁻ → bonds via S (thiocyanato) OR N (isothiocyanato)

Example: Like a servant who can shake hands with either their left OR right hand!

4. Chelation and Chelate Effect

What is Chelation?

When a ligand grabs the metal at TWO or MORE points, like a crab’s claw gripping something. The word “chelate” comes from Greek “chele” meaning claw!

    ╭─── Ligand ───╮
    │              │
    ↓              ↓
    ●──── Metal ────●

    The ligand forms a RING!

The Chelate Effect: Why Crabs Win

The Big Secret: Chelate complexes are MORE STABLE than similar complexes with monodentate ligands!

Comparison:

Even though both have 6 nitrogen atoms bonded to Ni, the one with “en” (a bidentate ligand forming rings) is way more stable!

Why Does This Happen?

Entropy is the hero!

When chelates form:

• 1 EDTA replaces 6 water molecules
• More particles are released
• More disorder (entropy) = more favorable

Simple Analogy: It’s easier to lose 6 separate keys than 1 keyring with all keys attached!

5. Macrocyclic Effect

Taking Chelation to the NEXT LEVEL

If chelate effect is good, macrocyclic effect is BETTER!

Macrocyclic ligands are ring-shaped molecules that completely surround the metal — like a crown on the king’s head!

graph TD
    A["Stability Increases"] --> B["Monodentate"]
    B --> C["Chelate"]
    C --> D["Macrocycle"]
    style D fill:#90EE90

Why Are Macrocycles Super Stable?

1. Pre-organized shape — the ligand is already shaped to fit the metal
2. No entropy loss — the ring is already formed
3. Perfect fit — like a lock and key!

Example: Crown ethers and porphyrins (found in hemoglobin!)

6. EAN Rule (Effective Atomic Number)

The 18-Electron Magic

Metals want to be like noble gases — stable and happy with 18 electrons in their outer shell!

The Rule:

Metal electrons + Ligand electrons = 18 (ideally)

How to Calculate EAN

EAN = Metal electrons + Electrons from ligands

Example: [Fe(CO)₅]

The compound is stable because it follows the EAN rule!

Another Example: [Cr(CO)₆]

Note: Not ALL compounds follow this rule strictly, but those that do are often very stable!

7. Naming Coordination Compounds

The Recipe for Names

Naming coordination compounds is like following a recipe. Here are the steps:

graph TD
    A["Naming Rules"] --> B["1. Cation before Anion"]
    B --> C["2. Ligands alphabetically"]
    C --> D["3. Metal last in complex"]
    D --> E["4. Oxidation state in Roman"]

The Complete Rules

1. Cation comes before anion (like regular salts)
2. Within the coordination sphere:
   • Ligands come BEFORE metal
   • Ligands in ALPHABETICAL order
   • Use prefixes: di-, tri-, tetra-, etc.
3. Metal name:
   • If complex is CATION → regular name + oxidation state
   • If complex is ANION → add “-ate” suffix + oxidation state

Ligand Name Changes

Examples

[Co(NH₃)₆]Cl₃ → Hexaamminecobalt(III) chloride

K₄[Fe(CN)₆] → Potassium hexacyanidoferrate(II)

[CoCl₂(NH₃)₄]Cl → Tetraamminedichloridocobalt(III) chloride

Trick: Alphabetical order ignores prefixes! So “dichloro” comes AFTER “ammine” (c > a)… wait, no! We compare the ligand name (chlorido vs ammine), and ammine (a) comes before chlorido ©!

8. Isomerism in Complexes

Same Recipe, Different Dishes!

Isomers are compounds with the same formula but different arrangements. Like using the same LEGO pieces to build different things!

graph TD
    I["Isomerism"] --> S["Structural Isomerism"]
    I --> ST["Stereoisomerism"]
    S --> S1["Ionization"]
    S --> S2["Hydrate/Solvate"]
    S --> S3["Linkage"]
    S --> S4["Coordination"]
    ST --> ST1["Geometrical"]
    ST --> ST2["Optical"]

Structural Isomerism

1. Ionization Isomerism Different ions inside vs outside the bracket

2. Hydrate (Solvate) Isomerism Water inside or outside?

• [Cr(H₂O)₆]Cl₃ (violet)
• [Cr(H₂O)₅Cl]Cl₂·H₂O (blue-green)
• [Cr(H₂O)₄Cl₂]Cl·2H₂O (green)

3. Linkage Isomerism Ambidentate ligands bonding differently

• [Co(NH₃)₅(NO₂)]²⁺ (nitro - N bonded, yellow)
• [Co(NH₃)₅(ONO)]²⁺ (nitrito - O bonded, red)

4. Coordination Isomerism Swapping ligands between metal centers

• [Co(NH₃)₆][Cr(CN)₆]
• [Cr(NH₃)₆][Co(CN)₆]

Stereoisomerism

1. Geometrical (cis-trans) Isomerism

In square planar [Pt(NH₃)₂Cl₂]:

     Cl        Cl              Cl        NH₃
      \      /                  \      /
       Pt                        Pt
      /      \                  /      \
    Cl        NH₃             NH₃       Cl

    cis-isomer               trans-isomer
   (same side)            (opposite sides)

2. Optical Isomerism

Mirror images that cannot be superimposed — like your left and right hands!

• Complexes with bidentate ligands often show this
• Example: [Co(en)₃]³⁺ has two mirror forms

Quick Summary: The Royal Court

You Made It!

Now you understand how metals make friends and build their royal courts! Coordination chemistry is everywhere — from the hemoglobin carrying oxygen in your blood to the catalysts making plastics. The metal-ligand bond is one of chemistry’s most beautiful partnerships!

Remember: The metal is the king, ligands are loyal servants, and together they create compounds that make life possible!