Isomerism in Coordination Compounds — 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., $NO_2^-$ via N or O). - Coordination: Ligand exchange between complex cation & anion (e.g., $[Co(NH_3)_6][Cr(CN)_6]$ vs $[Cr(NH_3)_6][Co(CN)_6]$). - Hydrate: Water as ligand $leftrightarrow$ lattice water (e.g., $[Cr(H_2O)_6]Cl_3$ vs $[Cr(H_2O)_5Cl]Cl_2 cdot H_2O$)

Stereoisomers: — Same connectivity, different spatial arrangement.

- Geometrical (cis-trans, fac-mer): - Square Planar ($MA_2B_2$): 2 isomers (cis, trans). - Octahedral ($MA_4B_2$): 2 isomers (cis, trans). - Octahedral ($MA_3B_3$): 2 isomers (fac, mer). - Optical (Enantiomers): Non-superimposable mirror images (chiral). - Octahedral $M(AA)_3$ (e.g., $[Co(en)_3]^{3+}$): Always chiral (2 enantiomers). - Octahedral cis-$M(AA)_2B_2$ (e.g., cis-$[Co(en)_2Cl_2]^+$): Chiral (2 enantiomers). Trans is achiral.

Chirality: — Absence of plane of symmetry and center of inversion.

2-Minute Revision

Isomerism in coordination compounds refers to complexes with the same chemical formula but different atomic arrangements. It's split into two main types: structural and stereoisomerism.

Structural Isomers differ in how atoms are connected. Key types include:

1
Ionization Isomerism: — Ligands and counter ions swap positions (e.g., $[Co(NH_3)_5Br]SO_4$ vs $[Co(NH_3)_5SO_4]Br$).
2
Linkage Isomerism: — Ambidentate ligands ($NO_2^-$, $SCN^-$) bind through different donor atoms (e.g., nitro vs nitrito).
3
Coordination Isomerism: — Ligands are exchanged between complex cationic and anionic parts (e.g., $[Co(NH_3)_6][Cr(CN)_6]$ vs $[Cr(NH_3)_6][Co(CN)_6]$).
4
Hydrate Isomerism: — Water molecules are either coordinated or in the lattice (e.g., $[Cr(H_2O)_6]Cl_3$ vs $[Cr(H_2O)_5Cl]Cl_2 cdot H_2O$).

Stereoisomers have the same connectivity but different spatial arrangements.

1
Geometrical Isomerism: — Ligands occupy different relative positions.

* Cis-trans: For $MA_2B_2$ (square planar) and $MA_4B_2$ (octahedral), identical ligands are adjacent (cis) or opposite (trans). * Fac-mer: For $MA_3B_3$ (octahedral), three identical ligands are on a face (fac) or along a meridian (mer).

1
Optical Isomerism: — Chiral complexes (non-superimposable mirror images) rotate plane-polarized light.

* Common in octahedral $M(AA)_3$ (e.g., $[Co(en)_3]^{3+}$) and cis-$M(AA)_2B_2$ complexes. Tetrahedral complexes are generally not chiral unless $MABCD$. Square planar complexes are rarely chiral.

5-Minute Revision

Isomerism is a cornerstone of coordination chemistry, explaining how compounds with identical chemical formulas can exhibit diverse properties due to differing atomic arrangements. This phenomenon is broadly categorized into structural and stereoisomerism.

Structural Isomerism involves differences in the actual connectivity of atoms or the composition of the coordination sphere. There are several subtypes:

Ionization Isomerism: — Here, a ligand inside the coordination sphere and a counter ion outside exchange positions. For example, $[Co(NH_3)_5Br]SO_4$ and $[Co(NH_3)_5SO_4]Br$ are ionization isomers. The former yields $SO_4^{2-}$ ions in solution, while the latter yields $Br^-$ ions.
Linkage Isomerism: — This occurs with ambidentate ligands, which can bind through two different donor atoms. The nitrite ion ($NO_2^-$) can bind via nitrogen (nitro, M-$NO_2$) or oxygen (nitrito, M-$ONO$). Similarly, thiocyanate ($SCN^-$) can bind via sulfur or nitrogen. These isomers often have distinct colors.
Coordination Isomerism: — Observed in compounds where both the cation and anion are complex ions. Ligands are exchanged between the two metal centers. For instance, $[Co(NH_3)_6][Cr(CN)_6]$ and $[Cr(NH_3)_6][Co(CN)_6]$ are coordination isomers.
Hydrate Isomerism: — A specific type of ionization isomerism where water molecules are involved, either as coordinated ligands or as lattice water. Example: $[Cr(H_2O)_6]Cl_3$ (violet) and $[Cr(H_2O)_5Cl]Cl_2 cdot H_2O$ (blue-green).

Stereoisomerism involves the same connectivity but different spatial arrangements of ligands.

Geometrical Isomerism: — Arises from different relative positions of ligands. It's common in square planar and octahedral complexes.

* **Square Planar ($MA_2B_2$ type):** Exhibits cis (identical ligands adjacent) and trans (identical ligands opposite) isomers. Example: $Pt(NH_3)_2Cl_2$ (cis-platin is anti-cancer, trans-platin is not).

* **Octahedral ($MA_4B_2$ type):** Also shows cis and trans isomers. Example: $[Co(NH_3)_4Cl_2]^+$ has cis and trans forms. * **Octahedral ($MA_3B_3$ type):** Exhibits facial (fac) and meridional (mer) isomers.

In fac, three identical ligands occupy one face of the octahedron; in mer, they lie along a meridian. Example: $[Co(NH_3)_3Cl_3]$.

Optical Isomerism: — Occurs when a complex is chiral (non-superimposable on its mirror image) and rotates plane-polarized light. These non-superimposable mirror images are called enantiomers.

* **Octahedral $M(AA)_3$ type:** Complexes with three symmetrical bidentate ligands (e.g., $[Co(en)_3]^{3+}$) are always chiral and exist as a pair of enantiomers ($Delta$ and $Lambda$ forms). * **Octahedral cis-$M(AA)_2B_2$ type:** The cis-isomer is chiral and optically active, while the trans-isomer typically has a plane of symmetry and is achiral.

Example: cis-$[Co(en)_2Cl_2]^+$ is optically active. * Tetrahedral complexes are generally not chiral unless all four ligands are different ($MABCD$). Square planar complexes are rarely chiral.

To master this, practice drawing structures, identifying symmetry elements, and applying definitions to various complex types.

Prelims Revision Notes

Isomerism in Coordination Compounds: NEET Revision Notes

I. Definition: Compounds with the same molecular formula but different arrangements of atoms.

II. Types of Isomerism:

A. Structural Isomerism (Constitutional Isomerism): Different connectivity of atoms. 1. Ionization Isomerism: Exchange of a ligand within the coordination sphere and a counter ion outside. * *Example:* $[Co(NH_3)_5Br]SO_4$ (yields $SO_4^{2-}$) and $[Co(NH_3)_5SO_4]Br$ (yields $Br^-$).

2. Linkage Isomerism: Ambidentate ligands bind through different donor atoms. * *Ambidentate Ligands:* $NO_2^-$ (nitro/nitrito), $SCN^-$ (thiocyanato/isothiocyanato), $CN^-$ (cyano/isocyano). * *Example:* $[Co(NH_3)_5NO_2]^{2+}$ (nitro) vs $[Co(NH_3)_5ONO]^{2+}$ (nitrito).

3. Coordination Isomerism: Ligand exchange between complex cation and complex anion. * *Example:* $[Co(NH_3)_6][Cr(CN)_6]$ vs $[Cr(NH_3)_6][Co(CN)_6]$. 4. Hydrate Isomerism (Solvate Isomerism): Specific type of ionization isomerism involving water molecules as ligands vs.

lattice water. * *Example:* $[Cr(H_2O)_6]Cl_3$ (6 coordinated $H_2O$) vs $[Cr(H_2O)_5Cl]Cl_2 cdot H_2O$ (5 coordinated $H_2O$, 1 lattice $H_2O$). 5. Ligand Isomerism: Isomeric ligands form isomeric complexes.

B. Stereoisomerism: Same connectivity, different spatial arrangement. 1. Geometrical Isomerism (cis-trans, fac-mer): * Square Planar (Coordination No. 4): * $MA_2B_2$ type (e.g., $Pt(NH_3)_2Cl_2$): 2 isomers (cis, trans).

* $MABCD$ type: 3 isomers. * Tetrahedral complexes generally *do not* show geometrical isomerism. * Octahedral (Coordination No. 6): * $MA_4B_2$ type (e.g., $[Co(NH_3)_4Cl_2]^+$): 2 isomers (cis, trans).

* $MA_3B_3$ type (e.g., $[Co(NH_3)_3Cl_3]$): 2 isomers (facial 'fac', meridional 'mer'). * $M(AA)_2B_2$ type (e.g., $[Co(en)_2Cl_2]^+$): 2 isomers (cis, trans). 2. Optical Isomerism (Enantiomerism): Chiral complexes (non-superimposable mirror images) rotate plane-polarized light.

* Conditions for Chirality: Absence of plane of symmetry and center of inversion. * Octahedral Complexes: * $M(AA)_3$ type (e.g., $[Co(en)_3]^{3+}$): Always chiral (2 enantiomers, $Delta$ and $Lambda$).

* cis-$M(AA)_2B_2$ type (e.g., cis-$[Co(en)_2Cl_2]^+$): Chiral (2 enantiomers). Trans-isomer is achiral. * $M(AA)B_2C_2$, $M(AA)B_2CD$, etc., can also be chiral. * Square Planar Complexes: Generally achiral.

Rare exceptions with specific unsymmetrical ligands. * Tetrahedral Complexes: Chiral if all four ligands are different ($MABCD$).

III. Key Points for NEET:

Memorize common examples for each isomer type.
Practice drawing/visualizing cis/trans and fac/mer structures.
Understand the conditions for optical activity (chirality).
Know the special cases (e.g., tetrahedral no geometrical, $M(AA)_3$ always optical).
Cisplatin is cis-$Pt(NH_3)_2Cl_2$ (anti-cancer).

Vyyuha Quick Recall

In London, Coordination Hydrates Generally Observe:

Ionization
Linkage
Coordination
Hydrate
Geometrical
Optical

This mnemonic helps remember the main types of isomerism in coordination compounds.