What is the primary difference between a coordination compound and a double salt?

The fundamental difference lies in their behavior in solution. A double salt, like Mohr's salt, completely dissociates into its constituent simple ions when dissolved in water, losing its identity. In contrast, a coordination compound, such as $\text{K}_4[\text{Fe}(\text{CN})_6]$, retains its identity in solution; only the counter ions dissociate, while the complex ion (the central metal and its directly bonded ligands) remains intact as a single unit. This stability of the complex ion in solution is the defining characteristic of a coordination compound.

How do I determine the coordination number of a central metal ion in a complex?

The coordination number is the total number of donor atoms directly bonded to the central metal ion. It's not simply the number of ligands. For monodentate ligands (e.g., $\text{NH}_3$, $\text{Cl}^-$), it's equal to the number of ligands. For bidentate ligands (e.g., 'en', 'ox'), each ligand contributes two donor atoms, so you multiply the number of bidentate ligands by two. For polydentate ligands, multiply by their denticity. For example, in $[\text{Co}(\text{en})_3]^{3+}$, 'en' is bidentate, so CN = $3 \times 2 = 6$.

What makes a ligand 'ambidentate'?

An ambidentate ligand is a monodentate ligand that possesses two different donor atoms, but it can only form a coordinate bond through one of these donor atoms at a time. It's like having two doors but only being able to enter through one at any given moment. Examples include $\text{NO}_2^-$ (can bond via N or O) and $\text{SCN}^-$ (can bond via S or N). This property leads to linkage isomerism in coordination compounds.

Explain the 'chelate effect' and its significance.

The chelate effect refers to the enhanced stability of a coordination complex containing chelating ligands (bidentate or polydentate ligands that form ring structures with the metal ion) compared to a similar complex with only monodentate ligands. This increased stability is primarily an entropic effect. When a chelating ligand replaces two or more monodentate ligands, the number of particles in the system often increases, leading to a greater disorder (higher entropy), which favors the formation of the chelate complex. This effect is crucial in biological systems and industrial applications.

How do I calculate the oxidation state of the central metal in a complex?

To calculate the oxidation state, first identify the charge of the overall complex ion (if counter ions are present, their charges will help determine this). Then, sum the known charges of all the ligands. Since the sum of the oxidation state of the metal and the charges of the ligands must equal the net charge of the complex ion, you can set up an algebraic equation. For example, in $[\text{Cr}(\text{H}_2\text{O})_4\text{Cl}_2]\text{Cl}$, the complex ion is $[\text{Cr}(\text{H}_2\text{O})_4\text{Cl}_2]^+$. $\text{H}_2\text{O}$ is neutral (0), $\text{Cl}^-$ is -1. Let Cr be $x$. So, $x + 4(0) + 2(-1) = +1 \implies x - 2 = +1 \implies x = +3$. The oxidation state of Cr is +3.

What is the difference between a homoleptic and a heteroleptic complex?

A homoleptic complex is one where the central metal atom or ion is bonded to only one type of ligand. For instance, in $[\text{Co}(\text{NH}_3)_6]^{3+}$, all six ligands are ammonia molecules. In contrast, a heteroleptic complex involves the central metal being bonded to two or more different types of ligands. An example is $[\text{Co}(\text{NH}_3)_4\text{Cl}_2]^+$, where both ammonia and chloride ions act as ligands. This distinction is important for understanding isomerism in coordination compounds.

Introduction and Terminology

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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Coordination compounds are a special class of compounds in which a central metal atom or ion is bonded to a fixed number of ions or molecules, known as ligands, by coordinate covalent bonds. These compounds retain their identity even when dissolved in a solvent, unlike double salts which dissociate into their constituent ions. The study of coordination compounds is crucial in various fields, inclu…

Quick Summary

Coordination compounds are complex substances where a central metal atom or ion is bonded to electron-donating species called ligands via coordinate covalent bonds. Unlike double salts, these compounds maintain their structural integrity in solution, with the central metal and its directly attached ligands forming a stable 'coordination sphere'.

Key terms include the central metal (a Lewis acid, typically a transition metal), ligands (Lewis bases, classified by denticity as monodentate, bidentate, or polydentate), coordination number (total donor atoms bonded to the metal), and oxidation state of the metal.

Chelating ligands form stable ring structures, exhibiting the chelate effect. Ambidentate ligands can bind through multiple donor atoms but only one at a time. Complexes can be homoleptic (one type of ligand) or heteroleptic (multiple ligand types).

Understanding these terms is fundamental for studying the structure, bonding, and reactivity of coordination compounds in NEET.

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Key Concepts

Ligand Denticity and Classification

Ligand denticity refers to the number of donor atoms a single ligand uses to bind to the central metal ion.…

Oxidation State Calculation

The oxidation state of the central metal ion is a hypothetical charge it would possess if all ligands were…

Coordination Sphere and Counter Ions

The coordination sphere, denoted by square brackets $[\dots]$ , represents the central metal ion and all the…

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]^+

)

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.