Structural and Stereoisomerism — Revision Notes

⚡ 30-Second Revision

Isomers: — Same formula, different arrangement.
Structural Isomers: — Different connectivity.

- Ionization: Ligand $leftrightarrow$ Counter-ion (e.g., $[Co(NH_3)_5Br]SO_4$ vs. $[Co(NH_3)_5SO_4]Br$). - Linkage: Ambidentate ligand binds via different atoms (e.g., $M-NO_2$ vs. $M-ONO$). - Hydrate: Coordinated $H_2O leftrightarrow$ Lattice $H_2O$ (e.g., $CrCl_3 cdot 6H_2O$ forms). - Coordination: Ligand exchange between complex cation & anion (e.g., $[Co(NH_3)_6][Cr(CN)_6]$ vs. $[Cr(NH_3)_6][Co(CN)_6]$).

Stereoisomers: — Same connectivity, different spatial arrangement.

- Geometrical (cis-trans): Different relative positions. - Square planar $MA_2B_2$: cis, trans. - Octahedral $MA_4B_2$: cis, trans. - Octahedral $MA_3B_3$: fac, mer. - Optical (Enantiomers): Non-superimposable mirror images (chiral). - Chiral if no plane/center of symmetry. - Examples: Tetrahedral $MABCD$, Octahedral $M(AA)_3$, $cis-M(AA)_2B_2$.

2-Minute Revision

Isomerism in coordination compounds means compounds have the same formula but different atom arrangements. It splits into two main types: structural and stereoisomerism.

Structural Isomerism involves differences in how ligands are connected to the central metal. Key types are:

1
Ionization Isomerism: — Ligands inside the coordination sphere swap with counter-ions outside, leading to different ions in solution (e.g., $[Co(NH_3)_5Br]SO_4$ vs. $[Co(NH_3)_5SO_4]Br$).
2
Linkage Isomerism: — Ambidentate ligands (like $NO_2^-$ or $SCN^-$) bind through different donor atoms (e.g., nitro vs. nitrito).
3
Hydrate Isomerism: — A specific type of ionization isomerism where water molecules are either coordinated or are lattice water.
4
Coordination Isomerism: — Ligands are exchanged between a complex cation and a complex anion (e.g., $[Co(NH_3)_6][Cr(CN)_6]$ vs. $[Cr(NH_3)_6][Co(CN)_6]$).

Stereoisomerism involves the same ligand connectivity but different spatial arrangements. It has two sub-types:

1
Geometrical Isomerism (cis-trans): — Ligands occupy different relative positions. Common in square planar $MA_2B_2$ (cis/trans) and octahedral $MA_4B_2$ (cis/trans) or $MA_3B_3$ (fac/mer).
2
Optical Isomerism: — Occurs when a complex is chiral (non-superimposable on its mirror image). This requires the absence of a plane of symmetry and a center of symmetry. Examples include octahedral $M(AA)_3$ (like $[Co(en)_3]^{3+}$) and $cis-M(AA)_2B_2$ complexes. Square planar complexes are generally optically inactive.

5-Minute Revision

Isomerism in coordination compounds is the phenomenon where compounds share the same chemical formula but differ in the arrangement of their ligands around the central metal ion, leading to distinct properties. This is broadly categorized into structural and stereoisomerism.

Structural Isomerism: These isomers differ in the actual connectivity of ligands to the metal. This means the chemical bonds formed are different.

Ionization Isomerism: — A ligand inside the coordination sphere and a counter-ion outside exchange positions. For example, $[Co(NH_3)_5Br]SO_4$ (releases $SO_4^{2-}$ ions) and $[Co(NH_3)_5SO_4]Br$ (releases $Br^-$ ions). They have different conductivities and precipitate different ions.
Linkage Isomerism: — Involves ambidentate ligands (e.g., $NO_2^-$, $SCN^-$) which can bind through two or more different donor atoms. For instance, $[Co(NH_3)_5(NO_2)]^{2+}$ (nitro, N-bonded) and $[Co(NH_3)_5(ONO)]^{2+}$ (nitrito, O-bonded) are linkage isomers, often differing in color.
Hydrate Isomerism: — A specific type of ionization isomerism where water molecules are either directly coordinated to the metal or exist as water of crystallization outside the coordination sphere. Example: $CrCl_3 cdot 6H_2O$ can exist as $[Cr(H_2O)_6]Cl_3$, $[Cr(H_2O)_5Cl]Cl_2 cdot H_2O$, and $[Cr(H_2O)_4Cl_2]Cl cdot 2H_2O$.
Coordination Isomerism: — Occurs in compounds where both cation and anion are complex ions, and ligands are exchanged between them. Example: $[Co(NH_3)_6][Cr(CN)_6]$ and $[Cr(NH_3)_6][Co(CN)_6]$.

Stereoisomerism: These isomers have the same connectivity but differ in the spatial arrangement of ligands.

Geometrical Isomerism (cis-trans): — Ligands occupy different relative positions.

* **Square Planar ($MA_2B_2$):** Two isomers: cis (identical ligands adjacent) and trans (identical ligands opposite). E.g., $cis-[Pt(NH_3)_2Cl_2]$ (cis-platin) and $trans-[Pt(NH_3)_2Cl_2]$. * **Octahedral ($MA_4B_2$):** Two isomers: cis (B ligands adjacent) and trans (B ligands opposite). E.g., $[Co(NH_3)_4Cl_2]^+$. * **Octahedral ($MA_3B_3$):** Two isomers: facial (fac, three identical ligands on a triangular face) and meridional (mer, three identical ligands in a plane).

Optical Isomerism (Enantiomerism): — Occurs when a complex is chiral – non-superimposable on its mirror image. Chiral molecules lack a plane of symmetry and a center of symmetry. Enantiomers rotate plane-polarized light in opposite directions.

* Tetrahedral: $MABCD$ type complexes are chiral. * Octahedral: $M(AA)_3$ type (e.g., $[Co(en)_3]^{3+}$) are always chiral. $cis-M(AA)_2B_2$ type (e.g., $cis-[Co(en)_2Cl_2]^+$) are chiral, while their trans counterparts are achiral. Visualizing 3D structures and identifying symmetry elements is crucial for optical isomerism.

Prelims Revision Notes

Structural and Stereoisomerism: NEET Quick Recall

I. Structural Isomerism (Constitutional Isomerism)

Definition: — Same molecular formula, different connectivity of ligands to metal.
Types:

* Ionization Isomerism: Ligand inside coordination sphere exchanges with counter-ion outside. * Test: Different ions produced in solution (e.g., $AgNO_3$ test for $Cl^-$, $BaCl_2$ for $SO_4^{2-}$).

* Example: $[Co(NH_3)_5Br]SO_4$ and $[Co(NH_3)_5SO_4]Br$. * Linkage Isomerism: Ambidentate ligand binds via different donor atoms. * Ambidentate Ligands: $NO_2^-$ (nitro/nitrito), $SCN^-$ (thiocyanato/isothiocyanato), $CN^-$ (cyano/isocyano).

* Example: $[Co(NH_3)_5(NO_2)]^{2+}$ (N-bonded) vs. $[Co(NH_3)_5(ONO)]^{2+}$ (O-bonded). * Hydrate Isomerism: Specific ionization isomerism involving $H_2O$. * Difference: Coordinated $H_2O$ vs.

lattice $H_2O$. * Example: $CrCl_3 cdot 6H_2O$ exists as $[Cr(H_2O)_6]Cl_3$, $[Cr(H_2O)_5Cl]Cl_2 cdot H_2O$, $[Cr(H_2O)_4Cl_2]Cl cdot 2H_2O$. * Coordination Isomerism: Ligands exchanged between complex cation and complex anion.

* Condition: Both cation and anion must be complex ions. * Example: $[Co(NH_3)_6][Cr(CN)_6]$ and $[Cr(NH_3)_6][Co(CN)_6]$.

II. Stereoisomerism

Definition: — Same molecular formula, same connectivity, different spatial arrangement.
Types:

* Geometrical Isomerism (cis-trans): Different relative positions of ligands. * Square Planar (Coordination No. 4): * $MA_2B_2$: cis (A-A adjacent), trans (A-A opposite). E.g., $[Pt(NH_3)_2Cl_2]$.

* $MABCD$: 3 isomers. * *Note:* Tetrahedral complexes generally don't show geometrical isomerism. * Octahedral (Coordination No. 6): * $MA_4B_2$: cis (B-B adjacent), trans (B-B opposite). E.g., $[Co(NH_3)_4Cl_2]^+$.

* $MA_3B_3$: facial (fac, ligands on a face), meridional (mer, ligands in a plane). E.g., $[Co(NH_3)_3Cl_3]$. * $M(AA)_2B_2$: cis (B-B adjacent), trans (B-B opposite). E.g., $[Co(en)_2Cl_2]^+$. * Optical Isomerism (Enantiomerism): Non-superimposable mirror images (chiral).

* Condition for Chirality: Absence of plane of symmetry and center of symmetry. * Optically Active: Rotates plane-polarized light. * Examples: * Tetrahedral $MABCD$. * Octahedral $M(AA)_3$ (e.

g., $[Co(en)_3]^{3+}$) - always chiral ($Delta$ and $Lambda$ forms). * Octahedral $cis-M(AA)_2B_2$ (e.g., $cis-[Co(en)_2Cl_2]^+$) - chiral. *Note: trans-M(AA)_2B_2 is achiral*. * Octahedral $cis-M(AA)_2BC$.

III. Key Points for NEET

Practice drawing structures for different geometries.
Systematically count isomers: structural first, then stereoisomers for each structural form.
Understand the biological significance of cis-platin.
Always check for symmetry elements (plane/center) to determine optical activity.

Vyyuha Quick Recall

In London, Hydrated Coordinators Get Outstanding Stereos.

Ionization
Linkage
Hydrate
Coordination
Geometrical
Optical
Structural (as a category for the first four)
Stereoisomerism (as a category for the last two)