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

3.4.5 · Chemistry › Coordination Chemistry

2. Structural Isomerism

2.1 Linkage isomerism

YEH KYUN HOTA HAI: Kuch ligands ke paas do atoms hote hain jinke paas lone pairs hote hain. Koi bhi metal ko donate kar sakta hai.

Classic ambidentate ligands:

Ligand	Donor atom A	Donor atom B
$N O_{2}^{-}$	N (nitro, –NO₂)	O (nitrito, –ONO)
$S C N^{-}$	S (thiocyanato-S)	N (isothiocyanato-N)
$C N^{-}$	C (cyano)	N (isocyano)

2.2 Ionization isomerism

KYA DHUNDHNA HAI: [brackets] wala ion bahar likhe ion se swap karta hai.

2.3 Hydrate (solvate) isomerism

Worked example Chromium chloride hexahydrate trio

$C r C l_{3} \cdot 6 H_{2} O$

Formula	Colour	Free Cl⁻ (AgNO₃ ke saath ppt deta hai)	$H_{2} S O_{4}$ ke upar lose hone wala water
$[C r (H_{2} O)_{6}] C l_{3}$	violet	3	0
$[C r (H_{2} O)_{5} C l] C l_{2} \cdot H_{2} O$	blue-green	2	1
$[C r (H_{2} O)_{4} C l_{2}] C l \cdot 2 H_{2} O$	green	1	2

Yeh step kyun? Sirf outside-sphere chloride $A g N O_{3}$ ke saath turant precipitate karta hai; sirf lattice water (coordinated water nahi) ko drying agent se remove kiya ja sakta hai. Inhe count karna aapko structure batata hai.

2.4 Coordination isomerism

3. Stereoisomerism

3.1 Geometrical (cis–trans) isomerism

Yeh kahan appear hota hai:

Square planar $M A_{2} B_{2}$ : cis (A's adjacent) aur trans (A's opposite). jaise $[P t (N H_{3})_{2} C l_{2}]$ .
Octahedral $M A_{4} B_{2}$ : cis aur trans.
Octahedral $M A_{3} B_{3}$ : fac (3 same ek triangular face par) aur mer (3 same ek meridian/plane mein).

3.2 Optical isomerism

Common chiral coordination motifs:

Octahedral with bidentate (chelating) ligands:
- $[M (e n)_{3}]^{n +}$ (jaise $[C o (e n)_{3}]^{3 +}$ ) → chiral ( $Δ$ aur $Λ$ forms).
- cis- $[M (e n)_{2} X_{2}]$ → chiral; trans- $[M (e n)_{2} X_{2}]$ → plane of symmetry hai → achiral.
Tetrahedral with 4 different ligands $M ab c d$ → chiral (rare, isolate karna mushkil).

4. Sab kuch ek saath (worked classification)

Recall Feynman: 12-saal ke bachche ko explain karo

LEGO imagine karo. Same number of bricks, alag builds = isomers.

Structural: aap actually bricks ko alag order mein snap karte ho (ek red brick jo andar thi woh bahar hai, ya ek special brick ulti hai). Connections alag hain.
Stereo: bricks same order mein snap hain, lekin aapne kuch pieces ko alag taraf point karne ke liye ghuma diya hai — jaise cis/trans (do flags ek doosre ke paas vs across facing), ya jaise apne left aur right hands jo identical lagte hain lekin perfectly stack nahi ho sakte (optical). Same parts, space mein alag shape → alag behaviour (ek cancer cure kar sakta hai, uska twin nahi!).

Flashcards

Structural aur stereoisomerism mein kya feature distinguish karta hai?

Structural = alag connectivity (kaun sa atom kahan bond karta hai); stereo = same connectivity, alag spatial arrangement.

Ek ambidentate ligand define karo.

Ek ligand jo do alag donor atoms ke through coordinate kar sakta hai (jaise NO₂⁻ N ya O ke through; SCN⁻ S ya N ke through).

[C o (N H_{3})_{5} (N O_{2})]^{2 +}

ke do linkage isomers aur unke colours batao.

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

Experimentally

[C o (N H_{3})_{5} B r] S O_{4}

[C o (N H_{3})_{5} S O_{4}] B r

se kaise distinguish karte hain?

Pehla free sulphate deta hai (BaCl₂ ke saath white ppt); doosra free bromide deta hai (AgNO₃ ke saath pale-yellow ppt).

Hydrate isomerism kya hai?

Aisa isomers jo differ karte hain ki kitne water molecules coordinated hain (inside sphere) vs crystallisation ka water (lattice), counter-ions accordingly adjust hote hain.

Tetrahedral

M A_{2} B_{2}

mein koi geometrical isomerism kyun NAHI hoti?

Tetrahedron ke saare four vertices equivalent/adjacent hote hain (109.5°); koi "trans" (opposite) position nahi hoti.

fac aur mer isomers mein kya fark hai?

Octahedral

M A_{3} B_{3}

mein: fac = teen identical ligands ek triangular face par; mer = teen identical ligands ek meridian ke around (planar arrangement).

Enantiomers define karo.

Non-superimposable mirror-image stereoisomers jo plane-polarised light ko equally lekin opposite directions mein rotate karte hain.

cis-/trans-

[C o (e n)_{2} C l_{2}]^{+}

mein se kaun optically active hai aur kyun?

cis chiral hai (koi symmetry plane nahi → Δ/Λ enantiomers ke roop mein exist karta hai); trans mein ek plane of symmetry hai → achiral.

Cisplatin active kyun hai lekin transplatin inactive kyun hai?

Sirf cis geometry do leaving Cl⁻ groups ko correctly place karta hai adjacent DNA bases se bind karne aur DNA ko distort karne ke liye.

Coordination isomerism kya hai?

Ek salt mein jahan cation aur anion dono complexes hain, ligands do metal centres ke beech alag tarah distribute hote hain, jaise

[C o (N H_{3})_{6}] [C r (C N)_{6}]

[C r (N H_{3})_{6}] [C o (C N)_{6}]

Ek complex mein optical activity ke liye quick test.

Mirror image draw karo; agar koi plane/centre of symmetry nahi hai aur woh non-superimposable hai, toh complex chiral hai.

Connections

Coordination Number and Geometry — geometry determine karta hai ki kaun se isomerism types possible hain.
Crystal Field Theory — kyun linkage isomers colour mein differ karte hain (alag $Δ_{o}$ ).
Werner's Theory and Coordination Sphere — sphere ke andar vs bahar ionization/hydrate isomerism ko underpin karta hai.
Chirality in Organic Chemistry — optical activity ke liye same plane-of-symmetry logic.
Cisplatin and Bioinorganic Chemistry — geometric isomerism with real biological stakes.
Ambidentate vs Polydentate Ligands — linkage isomerism / chelation ka source.