Question

Assertion: Chelates are less stable than ordinary coordination compounds.
Reason: Chelates can be formed by bidentate ligands.
(A)Both assertion and reason are correct and reason is the correct explanation for assertion.
(B)Both assertion and reason are correct but reason is not the correct explanation for assertion.
(C)Assertion is incorrect but reason is correct.
(D)Both assertion and reason are incorrect.

Answer 1

Hint : Coordination complexes are compounds that consist of one or more metal centers which are bonded to ligands and ions- cations or anions. Ligands are ions or molecules that donate a pair of electrons to the central metal atom. A chelate is formed when a polydentate ligand is bonded to the central metal atom.

Complete Step By Step Answer:
Ligands are ions or molecules bound to the central metal atom in the coordination entity. When it is bound through a single donor atom, like in $C{l^ - },{H_2}O,N{H_3}$ , then it is called unidentate ligand. If it can bind through two donor atoms of the same molecule as in ${H_2}NC{H_2}C{H_2}N{H_2}\,and\,{C_2}O_4^{2 - }$ ,it is called bidentate ligand and if there are more than two donor atoms in a molecule, then it is called polydentate. Didendate and polydentate ligands use their multiple donor atoms to bind to the metal center, forming chelate ligands. They can form cyclic complexes. Such chelate complexes are thermodynamically stable than similar complexes with unidentate ligands. Therefore, the given assertion is a wrong statement and the reason is correct.
The right option is (C)Assertion is incorrect but reason is correct.

Note :
The stabilization of coordination complexes due to chelation is called chelate effect.It is dependent on Gibb’s energy , entropy, enthalpy and is measured in terms of stepwise stability constant $(\kappa )$ or overall stability constant $(\beta )$ . Here, stability of complex refers to the degree of association between two species in equilibrium.