Coordination Compounds

🧪 COORDINATION COMPOUNDS — COPY-PASTE NOTES

1. Addition / Molecular Compounds

When solutions containing two or more simple stable compounds are mixed in molecular proportions and allowed to evaporate, crystals of a new compound may form.

These are called molecular or addition compounds.

Examples

  • CuSO₄ + 4NH₃ → CuSO₄·4NH₃

  • AgCN + KCN → KCN·AgCN

  • 4KCN + Fe(CN)₂ → Fe(CN)₂·4KCN

  • K₂SO₄ + Al₂(SO₄)₃ + 24H₂O → K₂SO₄·Al₂(SO₄)₃·24H₂O


2. Two types of addition compounds

A. Double salts / Lattice compounds

  • Stable in solid state.

  • When dissolved in water, they completely dissociate into individual constituent ions.

  • Therefore, their solution shows the properties/tests of all constituent ions.

Examples

Carnallite
KCl·MgCl₂·6H₂O

In water:
KCl·MgCl₂·6H₂O → K⁺ + Mg²⁺ + 3Cl⁻ + 6H₂O

Mohr’s salt
FeSO₄·(NH₄)₂SO₄·6H₂O

Alum
K₂SO₄·Al₂(SO₄)₃·24H₂O

Key difference

Double salt → completely dissociates → individual ions retain their identity.


3. Coordination / Complex Compounds

A coordination compound contains a complex ion/entity in which a central metal atom/ion is surrounded by ligands.

Example:

K₄[Fe(CN)₆]

In solution:

K₄[Fe(CN)₆] → 4K⁺ + [Fe(CN)₆]⁴⁻

The complex ion [Fe(CN)₆]⁴⁻ remains essentially intact.

Therefore, it does NOT give the usual tests for Fe²⁺ and CN⁻.

Complexes can be:

Perfect complexes

  • Fairly stable.

  • Not dissociated or only very slightly dissociated.

  • Example: [Fe(CN)₆]⁴⁻

Imperfect complexes

  • Less stable.

  • Reversibly dissociate enough to give tests of their constituent ions.

  • Example: [Cd(CN)₄]²⁻

Important

An extremely unstable imperfect complex may completely dissociate in solution and behave like a double salt.


4. Central ion / Central atom

The metal ion/atom to which one or more ligands are attached is called the central ion/central atom.

The central metal acts as an electron-pair acceptor.

Therefore, it must have vacant orbitals to accept electron pairs from ligands.

Example:

[Ni(NH₃)₆]²⁺ → central ion = Ni²⁺

[Fe(CN)₆]³⁻ → central ion = Fe³⁺


5. Ligands ⭐

A ligand is a neutral molecule, anion or cation directly attached to the central metal atom/ion in a complex.

Ligands donate a lone pair of electrons to the central metal.


The bond formed is a coordinate/dative bond.


Common ligands

Ligand

Formula

Charge

Aqua

Hâ‚‚O

0

Ammine

NH₃

0

Carbonyl

CO

0

Nitrosyl

NO

+1*

Fluoro

F⁻

−1

Chloro

Cl⁻

−1

Bromo

Br⁻

−1

Iodo

I⁻

−1

Hydroxo

OH⁻

−1

Cyano

CN⁻

−1

Oxalato

C₂O₄²⁻

−2

Sulphato

SO₄²⁻

−2

*NO can have special bonding/charge conventions in complexes, so use the context of the complex.


6. Classification of ligands

A. Based on charge

1. Anionic ligands

Negatively charged.

Examples:
F⁻, Cl⁻, CN⁻, S²⁻, SO₄²⁻

2. Neutral ligands

No charge.

Examples:
CO, NH₃, H₂O

3. Cationic ligands

Positively charged.

Examples:
NO⁺, NH₂NH₃⁺


7. Based on denticity ⭐⭐⭐

Denticity = number of coordinate bonds formed by ONE ligand with the central metal.

Monodentate / Unidentate

One donor atom → forms one coordinate bond.

Examples:
Cl⁻, H₂O, NH₃, NH₂⁻

Bidentate

Two donor atoms → forms two coordinate bonds.

Examples:

  • Ethylenediamine (en)

  • Oxalate (ox)

  • 1,10-phenanthroline

Tridentate

Three donor atoms.

Example:
Diethylenetriamine (dien)

Tetradentate

Four donor atoms.

Example:
Triethylenetetramine (trien)

Hexadentate

Six donor atoms.

Example:
EDTA⁴⁻


8. Chelating ligands ⭐

Ligands that form a ring with the central metal are called chelating ligands.

The ring formed is called a chelate ring.

All bidentate and polydentate ligands show chelation.

Examples:

  • en

  • oxalate

  • EDTA


9. Ambidentate ligands ⭐⭐⭐

Ligands that can coordinate through two different donor atoms, but only one at a time, are called ambidentate ligands.

Examples:

NO₂⁻

Can attach through:

  • N → nitro-N

  • O → nitrito-O

SCN⁻

Can attach through:

  • S → thiocyanato-S

  • N → thiocyanato-N

Important: Ambidentate ≠ bidentate.

An ambidentate ligand has two possible donor atoms but uses only one at a time.


10. Flexidentate ligand

A ligand having variable denticity, depending on the nature of the metal ion.

Example:

EDTA can show denticity 4 or 6.


11. Classification based on type of lone-pair donation

σ-donor

Donates a lone pair to form a σ bond.

Examples:
H₂O, NH₃

π-acceptor

  • Donates a lone pair to the metal through σ bonding.

  • Also accepts electron density from the metal into its vacant π* orbital.

Examples:
CO, NO

These are also called π-acid ligands.

π-donor

Can donate through pπ–dπ type interaction.

Examples given in the material include hydrocarbon π systems such as C₂H₄ and C₆H₆ in appropriate coordination contexts.


12. Coordination entity

A coordination entity consists of a central metal atom/ion bonded to a fixed number of ions or molecules through coordinate bonds.

Example:

[CoCl₃(NH₃)₃]

The entire bracketed species is the coordination entity.


13. Coordination number ⭐⭐⭐

Coordination number = number of coordinate bonds formed by ligands with the central metal ion/atom.

Examples:

[Ag(CN)₂]⁻

CN⁻ is monodentate.

2 CN⁻ → coordination number = 2

[Cu(NH₃)₄]²⁺

4 NH₃ → coordination number = 4

[Cr(H₂O)₆]³⁺

6 H₂O → coordination number = 6

[Co(en)₃]³⁺

Each en is bidentate.

3 × 2 = 6

Therefore coordination number = 6.

⚠ Do NOT simply count the number of ligand molecules. Count donor atoms/coordinate bonds.


14. Coordination polyhedron

The spatial arrangement of ligands around the central metal is called the coordination polyhedron.

Common geometries:

  • Coordination number 6 → Octahedral

  • Coordination number 4 → Tetrahedral / Square planar

  • Coordination number 5 → Trigonal bipyramidal / Square pyramidal


15. Oxidation number in complexes

Oxidation number is the charge on the central metal ion when the ligands are removed along with the electron pairs shared with the metal.

It is written in Roman numerals in brackets.

Example:

[Fe(CN)₆]³⁻

Let oxidation state of Fe = x.

x + 6(−1) = −3

x = +3

So Fe is Fe(III).


16. Homoleptic and heteroleptic complexes

Homoleptic

Contains only one type of ligand.

Example:

[Co(NH₃)₆]³⁺

Only NH₃ is present.

Heteroleptic

Contains more than one type of ligand.

Example:

[Co(NH₃)₄Cl₂]⁺

Contains NH₃ and Cl⁻.


17. Common ligand names you MUST know

Neutral ligands

H₂O → aqua

NH₃ → ammine

CO → carbonyl

NO → nitrosyl

CS → thiocarbonyl

Anionic ligands

F⁻ → fluoro

Cl⁻ → chloro

Br⁻ → bromo

I⁻ → iodo

CN⁻ → cyano

OH⁻ → hydroxo

NO₂⁻ → nitro/nitrito depending on bonding

NO₃⁻ → nitrato

SO₄²⁻ → sulphato

SO₃²⁻ → sulphito

C₂O₄²⁻ → oxalato

CH₃COO⁻ → acetato


18. Writing formula of coordination entities

Rules:

  1. Write the central atom first.

  2. Write ligands alphabetically.

  3. Order of ligands does not depend on their charge.

  4. Polyatomic ligands are generally written in parentheses.

  5. The entire coordination entity is enclosed in square brackets.

  6. No space between ligands and the metal.

  7. Charge is written outside the square bracket.

Example:

[Co(NH₃)₆]³⁺


19. Naming coordination compounds ⭐⭐⭐

Basic order

Ligands → central metal → oxidation state

For a complex ion:

[Co(NH₃)₆]³⁺

= hexaamminecobalt(III) ion

Greek prefixes

1 → mono
2 → di
3 → tri
4 → tetra
5 → penta
6 → hexa

For complicated ligand names, use:

bis, tris, tetrakis, etc.


20. Naming anionic complexes

If the complex ion is anionic, the metal name generally ends in -ate.

Examples:

Fe → ferrate

Cu → cuprate

Ag → argentate

Au → aurate

Pb → plumbate

Sn → stannate

Example:

K₄[Fe(CN)₆]

= Potassium hexacyanoferrate(II)


21. Werner’s Coordination Theory ⭐⭐⭐

Werner proposed that a coordination compound has two types of valencies.

Primary valency

= Oxidation state

  • Ionisable

  • Satisfied by anions

  • Non-directional

Secondary valency

= Coordination number

  • Non-ionisable

  • Satisfied by ligands

  • Directional

  • Determines geometry

Very important:

Primary valency → oxidation state

Secondary valency → coordination number


22. Werner’s examples

For:

CoCl₃·6NH₃

Modern formula:

[Co(NH₃)₆]Cl₃

Ionisation:

[Co(NH₃)₆]Cl₃ → [Co(NH₃)₆]³⁺ + 3Cl⁻

So:

  • Primary valency = 3

  • Secondary valency = 6


For:

CoCl₃·5NH₃

Modern formula:

[Co(NH₃)₅Cl]Cl₂

Ionisation gives:

[Co(NH₃)₅Cl]²⁺ + 2Cl⁻

So:

  • Primary valency = 3

  • Secondary valency = 6


23. AgNO₃ test and Werner theory

AgNO₃ precipitates free Cl⁻ ions as AgCl.

Therefore:

Number of moles of AgCl formed = number of ionisable Cl⁻ ions outside the coordination sphere.

Examples:

[Co(NH₃)₆]Cl₃ → 3 AgCl

[Co(NH₃)₅Cl]Cl₂ → 2 AgCl

[Co(NH₃)₄Cl₂]Cl → 1 AgCl

[Co(NH₃)₃Cl₃] → 0 AgCl

This is VERY important for JEE.


24. Effective Atomic Number (EAN) ⭐⭐⭐

Proposed by Sidgwick.

EAN = effective number of electrons around the central metal after receiving electron pairs from ligands.

Formula:

EAN = Atomic number − Oxidation state + 2 × Coordination number

Example:

For [Fe(CN)₆]⁴⁻

Fe atomic number = 26
Oxidation state = +2
Coordination number = 6

EAN = 26 − 2 + 2(6)

EAN = 36

36 = atomic number of Kr.


25. EAN examples

Ni(CO)₄

Ni = 28
O.S. = 0
C.N. = 4

EAN = 28 − 0 + 2(4)

= 36

[Co(NH₃)₆]³⁺

Co = 27
O.S. = +3
C.N. = 6

EAN = 27 − 3 + 12

= 36

[Cu(NH₃)₄]²⁺

Cu = 29
O.S. = +2
C.N. = 4

EAN = 29 − 2 + 8

= 35

⚠ EAN rule is not universally obeyed.