Question

Which is not true about a ligand?

a) It can act as Lewis base

b) It can be monodentate or multidentate

c) A monodentate ligand cannot be chelating ligand

d) A multidentate ligand cannot cause chelation

Answer 1

A ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. And a chelating ligand is a ligand that is attached to a central metal ion by bonds from two or more donor atoms. Whereas a Lewis base is any substance, such as the $OH^-$ ion, that can donate a pair of nonbonding electrons. So, Lewis base is therefore an electron-pair donor. By considering these properties we will look into these options and solve the problem.

Complete answer:

We were given a ligand and we asked to know its properties. So, let's analyze the options and look into this. A coordination complex is the product of a Lewis acid-base reaction in which neutral molecules or anions called ligands bond to a central metal atom or ion by coordinating covalent bonds. Hence option ‘a’ is correct. Ligand can be monodentate and multidentate as well. chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central atom. Hence multidentate ligand cannot cause chelation is the wrong answer.

Hence, option (d) is correct answer.

Note:

Experimentally, it is observed that metal complexes of polydentate ligands are significantly more stable than the corresponding complexes of chemically similar monodentate ligands; this is due to the fact that this has a chelate effect. Hence usually the stability of a chelate complex depends on the size of the chelate rings such as for ligands with a flexible organic backbone like ethylene diamine, complexes that contain five-membered chelate rings, which have almost no strain, are known to have much more stability than complexes with six-membered chelate rings.