Question

What's ligand and how are they classified​

Answer 1

Explanation:

Ligands: In co-ordination chemistry ligands are ion, molecule or any species which donates electron pair to central metal atom.

Depending on the type of interaction Ligands are of three types.

1. Sigma donor only
2. sigma as well as pi donor
3. pi acceptor ligand

let's understand each type of Ligands individually & in more detail.

1 - Sigma donor only: This is a unidirectional interaction, in which filled ligand overlaps (head to head) with central metal atom/ion & donates pair of electron in the LUMO of metal.

generally all the molecules of 2nd period without pi bond comes in this category, below are few example of sigma donor ligands,

$\small \sf NH_3, H_2O, CH_3^-, H^-, R-OH, R-NH_3, etc$

2- Pi donor: This in also a unidirectional interaction between ligand & central metal atom but the along with head to head overlap, side overlapping takes place.

generally protonated neutral molecules who have more than one pair to donate show such interaction, for e.g.

NH3 have two lone pair to donate but the energy level of both the lone pairs are different hence when it is neutral it only donates one pair of electron. but when NH3 is protonated to NH2- it have two electron pairs (negative charge+ lone pair) to donate & both the pairs have same energy level. example of such ligands are below,

$\sf \small NH_2^-, OH^-, R-O^-, R-NH^-, F^-, Cl^-, Br^- SH^- etc$

3- Pi acceptor ligand: This is a bidirectional interaction between ligand & central metal atom/ion, the filled orbital of ligand undergoes head to head to overlap with vacant orbital of central metal atom, & filled D orbital of central metal donates their pair to vacant LUMO of ligand.

depending on the LUMO pi acceptor ligands are further classified into two categories.

$d\pi - \sigma* \small \sf When \: lumo \: is \: \sigma*\\ d\pi - \pi* \small \: \sf When \: lumo \: is \: \pi*$

The dπ-σ* is seen in molecules of 3rd period onwards without pi bond for e.g.

PH3, PR3, AsR3 & SR2 etc

The dπ-π* is seen in molecules of 2nd or3rd period with pi bond for e.g.

CO C N- SC N^- etc

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