Isomerism — structural (linkage, ionization, coordination, hydrate) and stereo (geometrical, optical)

2. Structural Isomerism

2.1 Linkage isomerism

WHY it happens: Some ligands have two atoms with lone pairs. Either can donate to the metal.

Classic ambidentate ligands:

Ligand	Donor atom A	Donor atom B
$NO_2^-$	N (nitro, –NO₂)	O (nitrito, –ONO)
$SCN^-$	S (thiocyanato-S)	N (isothiocyanato-N)
$CN^-$	C (cyano)	N (isocyano)

2.2 Ionization isomerism

WHAT to look for: the ion in [brackets] swaps with the ion written outside.

2.3 Hydrate (solvate) isomerism

Worked example The chromium chloride hexahydrate trio

$CrCl_3\cdot 6H_2O$

Formula	Colour	Cl⁻ free (gives ppt with AgNO₃)	Water lost over $H_2SO_4$
$[Cr(H_2O)_6]Cl_3$	violet	3	0
$[Cr(H_2O)_5Cl]Cl_2\cdot H_2O$	blue-green	2	1
$[Cr(H_2O)_4Cl_2]Cl\cdot 2H_2O$	green	1	2

Why this step? Only outside-sphere chloride precipitates instantly with $AgNO_3$ ; only lattice water (not coordinated water) can be removed by a drying agent. Counting these tells you the structure.

2.4 Coordination isomerism

3. Stereoisomerism

3.1 Geometrical (cis–trans) isomerism

Where it appears:

Square planar $MA_2B_2$ : cis (A's adjacent) and trans (A's opposite). e.g. $[Pt(NH_3)_2Cl_2]$ .
Octahedral $MA_4B_2$ : cis and trans.
Octahedral $MA_3B_3$ : fac (3 same on one triangular face) and mer (3 same in a meridian/plane).

3.2 Optical isomerism

Common chiral coordination motifs:

Octahedral with bidentate (chelating) ligands:
- $[M(en)_3]^{n+}$ (e.g. $[Co(en)_3]^{3+}$ ) → chiral ( $\Delta$ and $\Lambda$ forms).
- cis- $[M(en)_2X_2]$ → chiral; trans- $[M(en)_2X_2]$ → has a plane of symmetry → achiral.
Tetrahedral with 4 different ligands $Mabcd$ → chiral (rare, hard to isolate).

4. Putting it together (worked classification)

Recall Feynman: explain to a 12-year-old

Imagine LEGO. Same number of bricks, different builds = isomers.

Structural: you actually snap the bricks together in a different order (a red brick that was inside is now outside, or a special brick is flipped around). The connections are different.
Stereo: the bricks are snapped in the same order, but you've turned some pieces so they point a different way — like cis/trans (two flags next to each other vs facing across), or like your left and right hands which look identical but can't stack perfectly (optical). Same parts, different shape in space → different behaviour (one can cure cancer, its twin can't!).

Flashcards

What feature distinguishes structural from stereoisomerism?

Structural = different connectivity (which atom bonds where); stereo = same connectivity, different spatial arrangement.

Define an ambidentate ligand.

A ligand that can coordinate through two different donor atoms (e.g. NO₂⁻ via N or O; SCN⁻ via S or N).

Name the two linkage isomers of

[Co(NH_3)_5(NO_2)]^{2+}

and their colours.

Nitro (N-bonded, yellow) and nitrito (O-bonded, red).

How do you experimentally distinguish

[Co(NH_3)_5Br]SO_4

from

[Co(NH_3)_5SO_4]Br

First gives free sulphate (white ppt with BaCl₂); second gives free bromide (pale-yellow ppt with AgNO₃).

What is hydrate isomerism?

Isomers differing in how many water molecules are coordinated (inside sphere) vs water of crystallisation (lattice), with counter-ions adjusting accordingly.

Why does tetrahedral

MA_2B_2

show NO geometrical isomerism?

All four vertices of a tetrahedron are equivalent/adjacent (109.5°); there is no "trans" (opposite) position.

What is the difference between fac and mer isomers?

In octahedral

MA_3B_3

: fac = three identical ligands on one triangular face; mer = three identical ligands around a meridian (planar arrangement).

Define enantiomers.

Non-superimposable mirror-image stereoisomers that rotate plane-polarised light equally but in opposite directions.

Which of cis-/trans-

[Co(en)_2Cl_2]^+

is optically active and why?

cis is chiral (no symmetry plane → exists as Δ/Λ enantiomers); trans has a plane of symmetry → achiral.

Why is cisplatin active but transplatin inactive?

Only the cis geometry places the two leaving Cl⁻ groups correctly to bind adjacent DNA bases and distort DNA.

What is coordination isomerism?

In a salt where both cation and anion are complexes, ligands are distributed differently between the two metal centres, e.g.

[Co(NH_3)_6][Cr(CN)_6]

[Cr(NH_3)_6][Co(CN)_6]

Quick test for optical activity in a complex.

Draw mirror image; if no plane/centre of symmetry exists and it's non-superimposable, the complex is chiral.

Connections

Coordination Number and Geometry — geometry determines which isomerism types are possible.
Crystal Field Theory — why linkage isomers differ in colour (different $\Delta_o$ ).
Werner's Theory and Coordination Sphere — inside vs outside the sphere underpins ionization/hydrate isomerism.
Chirality in Organic Chemistry — same plane-of-symmetry logic for optical activity.
Cisplatin and Bioinorganic Chemistry — geometric isomerism with real biological stakes.
Ambidentate vs Polydentate Ligands — source of linkage isomerism / chelation.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, isomerism ka matlab simple hai: molecular formula same, par arrangement alag — isliye properties (rang, reactivity, conductivity) bilkul different. Coordination chemistry me do bade family hote hain. Structural isomerism me atoms ka connection hi badal jata hai — kaun atom kis se juda hai. Isme chaar type yaad rakho "LICH": Linkage (ambidentate ligand jaise NO₂⁻ N se ya O se bind kare), Ionization (sphere ke andar ka ligand bahar wale counter-ion se swap ho — jaise [Co(NH₃)₅Br]SO₄ vs [Co(NH₃)₅SO₄]Br), Coordination (jab cation aur anion dono complex ho, ligand dono metal me bat-te hain), aur Hydrate (paani andar coordinate ho ya bahar lattice me — CrCl₃·6H₂O ke teen rang isi se aate hain).

Stereoisomerism me bonds same rehte hain, sirf space me direction alag hoti hai — yaad rakho "GO": Geometrical aur Optical. Geometrical me cis (do same ligand pass-pass, 90°) aur **tr

Isomerism — structural (linkage, ionization, coordination, hydrate) and stereo (geometrical, optical)

1. The master map

2. Structural Isomerism

2.1 Linkage isomerism

2.2 Ionization isomerism

2.3 Hydrate (solvate) isomerism

2.4 Coordination isomerism

3. Stereoisomerism

3.1 Geometrical (cis–trans) isomerism

3.2 Optical isomerism

4. Putting it together (worked classification)

Flashcards

Connections

Concept Map

Hinglish (regional understanding)

Go deeper — visual, from zero

Test yourself — Coordination Chemistry

Connections