Maharashtra Board Class 12 Chemistry Chapter 9 Coordination Compounds PDF Download

Coordination Compounds

9.1 Introduction

A coordination compound consists of a central metal atom or ion surrounded by atoms or molecules.

For example, a chemotherapy drug, cisplatin, \(\text{Pt(NH}_3\text{)}_2\text{Cl}_2\), is a coordination compound in which the central platinum metal ion is surrounded by two ammonia molecules and two chloride ions.

The species surrounding the central metal atom or ion are called ligands.

The ligands are linked directly to the central metal ion through coordinate bonds.

A formation of a coordinate bond occurs when the shared electron pair is contributed by ligands.

A coordinate bond is conveniently represented by an arrow, where the arrow head points to the electron acceptor.

The central metal atom or ion, usually an electron deficient species, accepts an electron pair while the ligands serve as electron donors.

Coordination compounds having a metal ion in the centre are common.

In cisplatin, two ammonia molecules and two chloride ligands utilize their lone pairs of electrons to form bonds with the Pt(II).

The donor nitrogen and chlorine atoms of the ligands are directly attached to and form bonds with platinum.

Formation of a coordination compound can be looked upon as the Lewis acid-base interaction.

The ligands being electron pair donors are Lewis bases.

The central metal ion being electron pair acceptor serves as Lewis acid.

Teacher's Note

Coordination compounds are very common in nature. For example, the red colour in blood comes from iron in haemoglobin, which is a coordination compound.

Exam Trick

Remember: Ligand = Electron Donor. Metal = Electron Acceptor. Just like in a battery, one side gives electrons and one side takes electrons.

Points to Remember

Coordination compounds have a central metal atom surrounded by ligands.
Ligands are atoms or molecules that donate electron pairs to the metal.
A coordinate bond forms when a ligand gives electrons to the metal.
The metal acts as a Lewis acid and the ligand acts as a Lewis base.

9.2 Types of Ligands

The ligands can be classified as monodentate and polydentate ligands depending on the number of electron donor atoms they have.

9.2.1 Monodentate Ligands

A monodentate ligand is the one where a single donor atom shares an electron pair to form a coordinate bond with the central metal ion.

For example, the ligands \(\text{Cl}^-\), \(\text{OH}^-\), or \(\text{CN}^-\) attached to metal have electron pair on Cl, O, and N, respectively, which are donor atoms.

Teacher's Note

Monodentate means "one tooth". Think of it like a single nail holding something. Ammonia is a common monodentate ligand.

Exam Trick

Remember: Mono = One. Monodentate ligand has one donor atom. Just like monorail has one rail, monodentate has one connection point.

Points to Remember

Monodentate ligands have only one electron donor atom.
Examples include Cl-, OH-, and CN-.
Each monodentate ligand forms only one coordinate bond with the metal.
Ammonia (NH3) and water (H2O) are neutral monodentate ligands.

9.2.2 Polydentate Ligands

A polydentate ligand has two or more donor atoms linked to the central metal ion.

For example, ethylenediamine and oxalate ion.

Each of these ligands possesses two donor atoms.

These are bidentate ligands.

Ethylenediamine binds to metal using electron pair on each of its two nitrogens.

Similarly, oxalate ion \(\text{(C}_2\text{O}_4\text{)}_2^-\) utilizes electron pair on each of its negatively charged oxygen atoms upon linking with the metal.

Ethylenediaminetetraacetate ion (EDTA)\(^{4-}\) binds to metal ion by electron pairs of four oxygen and two nitrogen atoms.

It is a hexadentate ligand.

Teacher's Note

Polydentate means "many teeth". Ethylenediamine (en) is used in many lab experiments because it can hold metals very strongly with its two connection points.

Exam Trick

Remember: Bidentate = Two teeth. Hexadentate = Six teeth. More donor atoms mean stronger binding to the metal, like more nails holding something down more tightly.

Points to Remember

Polydentate ligands have two or more donor atoms.
Bidentate ligands have two donor atoms and form two coordinate bonds.
Hexadentate ligands have six donor atoms and form six coordinate bonds.
EDTA is a hexadentate ligand used in medicine to remove poisons from the body.

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