Introduction and Terminology — Revision Notes

⚡ 30-Second Revision

Central Metal — Lewis acid (e.g., $\text{Fe}^{2+}$, $\text{Co}^{3+}$)
Ligand — Lewis base (e.g., $\text{NH}_3$, $\text{Cl}^-$, $\text{en}$)
Coordinate Bond — Ligand donates electron pair to metal.
Coordination Number (CN) — Number of donor atoms bonded to metal.

* Monodentate: 1 donor (e.g., $\text{H}_2\text{O}$, $\text{NH}_3$, $\text{Cl}^-$) * Bidentate: 2 donors (e.g., $\text{en}$, $\text{ox}^{2-}$) * Polydentate: >2 donors (e.g., $\text{EDTA}^{4-}$)

Coordination Sphere — $[\text{Metal}(\text{Ligand})_n]^{\pm}$ (non-dissociating unit)
Counter Ions — Ions outside $[\dots]$ (dissociate in solution)
Oxidation State — Charge on metal (calculate: $x + \sum(\text{ligand charges}) = \text{complex charge}$)
Chelating Ligand — Bidentate/polydentate, forms ring (e.g., $\text{en}$, $\text{EDTA}^{4-}$). Leads to chelate effect (increased stability).
Ambidentate Ligand — Monodentate, 2 different donor atoms, binds via one at a time (e.g., $\text{NO}_2^-$, $\text{SCN}^-$).
Homoleptic — One type of ligand (e.g., $[\text{Co}(\text{NH}_3)_6]^{3+}$)
Heteroleptic — Multiple types of ligands (e.g., $[\text{Co}(\text{NH}_3)_4\text{Cl}_2]^+$)

2-Minute Revision

Coordination compounds are distinct from double salts because they maintain their complex ion identity in solution. At their core is a central metal atom/ion (a Lewis acid, typically a transition metal) surrounded by ligands (Lewis bases, electron donors).

Ligands are classified by their denticity: monodentate (one donor atom, e.g., $\text{NH}_3$), bidentate (two donor atoms, e.g., ethylenediamine 'en'), or polydentate (multiple donor atoms, e.g., EDTA).

The coordination number is the total number of donor atoms directly bonded to the metal. The coordination sphere is the metal and its directly attached ligands, enclosed in square brackets, which acts as a single unit.

Counter ions balance the complex's charge but are outside the sphere and dissociate. The oxidation state of the central metal is calculated by balancing charges. Chelating ligands form stable ring structures, leading to the chelate effect.

Ambidentate ligands are monodentate but can bind through two different atoms (e.g., $\text{NO}_2^-$). Complexes can be homoleptic (one ligand type) or heteroleptic (multiple ligand types).

Mastering these terms is crucial for NEET, as they form the basis for understanding isomerism, bonding, and properties.

5-Minute Revision

Coordination compounds are fascinating chemical entities where a central metal atom or ion forms coordinate covalent bonds with surrounding electron-donating species called ligands. Unlike double salts, which fully dissociate in solution, coordination compounds retain their structural integrity as a complex ion (the central metal and its directly bound ligands, known as the coordination sphere). Any ions outside this sphere are counter ions and dissociate.

Key Components & Concepts:

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Central Metal Atom/Ion — Always a Lewis acid (electron pair acceptor), typically a d-block element due to available vacant d-orbitals. Its oxidation state is a critical parameter, calculated by summing ligand charges and equating to the complex ion's net charge. For example, in $[\text{Co}(\text{NH}_3)_6]\text{Cl}_3$, the complex ion is $[\text{Co}(\text{NH}_3)_6]^{3+}$. Since $\text{NH}_3$ is neutral, Co's oxidation state is +3.
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Ligands — Lewis bases (electron pair donors) that can be neutral (e.g., $\text{H}_2\text{O}$, $\text{NH}_3$, $\text{CO}$), anionic (e.g., $\text{Cl}^-$, $\text{CN}^-$, $\text{C}_2\text{O}_4^{2-}$), or rarely cationic. They are classified by denticity:

* Monodentate: One donor atom (e.g., $\text{NH}_3$, $\text{Cl}^-$). * Bidentate: Two donor atoms (e.g., ethylenediamine 'en', oxalate 'ox'). * Polydentate: More than two donor atoms (e.g.

, EDTA, hexadentate). * Chelating Ligands: Bidentate or polydentate ligands that form stable ring structures with the metal, exhibiting the chelate effect (enhanced stability due to entropy).

* Ambidentate Ligands: Monodentate ligands with two different potential donor atoms, binding through only one at a time (e.g., $\text{NO}_2^-$ can bind via N or O; $\text{SCN}^-$ via S or N).

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Coordination Number (CN) — The total number of donor atoms directly attached to the central metal. For $[\text{Co}(\text{en})_3]^{3+}$, 'en' is bidentate, so CN = $3 \times 2 = 6$.
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Complex Types — Homoleptic complexes have only one type of ligand (e.g., $[\text{Ni}(\text{CO})_4]$), while heteroleptic complexes have multiple types (e.g., $[\text{Pt}(\text{NH}_3)_2\text{Cl}_2]$).

NEET Focus: Practice identifying all these components from a given formula, calculating oxidation states and coordination numbers, and distinguishing between different ligand types. Understanding the chelate effect and ambidentate ligands is particularly important as they lead to isomerism.

Prelims Revision Notes

Coordination Compounds: Introduction & Terminology (NEET Revision)

1. Coordination Compounds vs. Double Salts:

Coordination Compounds — Retain identity in solution. Complex ion (metal + ligands) remains intact. E.g., $\text{K}_4[\text{Fe}(\text{CN})_6]$ yields $\text{K}^+$ and $[\text{Fe}(\text{CN})_6]^{4-}$.
Double Salts — Dissociate completely into simple ions in solution. Lose identity. E.g., Mohr's salt $\text{FeSO}_4 \cdot (\text{NH}_4)_2\text{SO}_4 \cdot 6\text{H}_2\text{O}$ yields $\text{Fe}^{2+}$, $\text{NH}_4^+$, $\text{SO}_4^{2-}$.

2. Key Terminology:

Central Metal Atom/Ion — Lewis acid (electron acceptor). Typically d-block elements. E.g., $\text{Co}^{3+}$, $\text{Ni}^{2+}$.
Ligands — Lewis base (electron donor). Molecules or ions with lone pairs. E.g., $\text{NH}_3$, $\text{H}_2\text{O}$, $\text{Cl}^-$, $\text{CN}^-$.

* Monodentate: 1 donor atom. E.g., $\text{NH}_3$ (ammine, 0), $\text{H}_2\text{O}$ (aqua, 0), $\text{Cl}^-$ (chloro, -1), $\text{CN}^-$ (cyano, -1), $\text{CO}$ (carbonyl, 0), $\text{NO}_2^-$ (nitro, -1).

* Bidentate: 2 donor atoms. E.g., Ethylenediamine (en, $\text{H}_2\text{N}-\text{CH}_2-\text{CH}_2-\text{NH}_2$, 0), Oxalate ($\text{C}_2\text{O}_4^{2-}$, ox, -2). * Polydentate: >2 donor atoms.

E.g., EDTA$^{4-}$ (hexadentate, -4). * Chelating Ligands: Bidentate/polydentate ligands forming stable ring structures (chelate rings). Exhibit chelate effect (increased stability due to entropy).

* Ambidentate Ligands: Monodentate ligands with two different donor atoms, binding via one at a time. E.g., $\text{NO}_2^-$ (nitro/nitrito), $\text{SCN}^-$ (thiocyanato/isothiocyanato).

Coordination Number (CN) — Total number of donor atoms directly bonded to the central metal. For $[\text{Co}(\text{en})_3]^{3+}$, CN = $3 \times 2 = 6$.
Coordination Sphere — Central metal + directly attached ligands, enclosed in $[\dots]$. Acts as a single unit.
Complex Ion — Coordination sphere with a net charge. E.g., $[\text{Cu}(\text{NH}_3)_4]^{2+}$.
Counter Ions — Ions outside the coordination sphere, balancing charge. Dissociate in solution. E.g., $\text{Cl}^-$ in $[\text{Co}(\text{NH}_3)_6]\text{Cl}_3$.
Oxidation State of Central Metal — Calculate using overall complex charge and known ligand charges. E.g., in $[\text{Fe}(\text{CN})_6]^{4-}$, $x + 6(-1) = -4 \implies x = +2$.
Homoleptic Complex — Only one type of ligand. E.g., $[\text{Ni}(\text{CO})_4]$.
Heteroleptic Complex — More than one type of ligand. E.g., $[\text{Pt}(\text{NH}_3)_2\text{Cl}_2]$.

3. Common Ligand Charges to Memorize:

Neutral (0): $\text{NH}_3$, $\text{H}_2\text{O}$, $\text{CO}$, $\text{NO}$, $\text{en}$, $\text{py}$ (pyridine)
Anionic (-1): $\text{Cl}^-$, $\text{Br}^-$, $\text{I}^-$, $\text{F}^-$, $\text{CN}^-$, $\text{OH}^-$, $\text{NO}_2^-$, $\text{SCN}^-$
Anionic (-2): $\text{O}^{2-}$, $\text{SO}_4^{2-}$, $\text{C}_2\text{O}_4^{2-}$ (oxalate)
Anionic (-4): $\text{EDTA}^{4-}$

Vyyuha Quick Recall

To remember types of ligands and their denticity, think of 'MAD BC P':

Mono-dentate: Ammine, Dichloro ($\text{NH}_3$, $\text{Cl}^-$)
Bi-dentate: Chelating (e.g., Ethylenediamine 'en', Oxalate 'ox')
Poly-dentate: (e.g., EDTA)

For Ambidentate Ligands, remember 'NO SCN':

Nitrite ($\text{NO}_2^-$) can bind via N or O.
Sulfocyanide ($\text{SCN}^-$) can bind via S or N.

This helps quickly recall common examples and their key characteristics.