Question

The following question is a case-based question. Read the case carefully and answer the questions that follow: 

The oxidation number of the central atom in a complex is defined as the charge it would carry if all the ligands are removed along with the electron pairs that are shared with the central atom. Similarly, the charge on the complex is the sum of the charges of the constituent parts, i.e., the sum of the charges on the central metal ion and its surrounding ligands. Based on this, the complex is called neutral if the sum of the charges of the constituents is equal to zero. However, for an anion or cationic complex, the sum of the charges of the constituents is equal to the charge on the coordination sphere.

Based on the above information, answer the following questions:

(a) Define ambidentate ligand with an example.  (1)

(b) What type of isomerism is shown by \[\ce{[CO(NH3)5Cl]SO4}\] and \[\ce{[Co(NH3)5SO4]Cl}\]?  (1)

(c) Define Chelate effect. How it affects the stability of complex?  (2)

                                              OR

(c) Find the coordination number and oxidation state of chromium in \[\ce{Na3[Cr(C2O4)3]}\].  (2)

Answer 1

(a) A ligand which has two different donor atoms and either of the two sites in the ligand can form coordinate bonds with the central atom is called an ambidentate ligand.

Ex: \[\ce{NO^-_2}\], \[\ce{SCN-}\], \[\ce{CN-}\]

(b) Ionisation isomerism

(C) The stabilization of coordination compounds due to metal chelate formation is known as the "chelate effect". Because entropy drops during chelation, the process becomes more favourable, and a ring complex known as a chelate ring is formed. The chelating ligands produce more stable complexes than the unidentate analogies. 

                                            OR

(C) In complex \[\ce{Na3[Cr(C2O4)3]}\] the coordination number of Cr is 6, as oxalate is a bidentate ligand.

Coordination no. = No. of ligands × denticity of ligand

= 3 × 2

= 6