Question

The coordination number of $Cr$ in ${K_3}\left[ {Cr{{\left( {{C_2}{O_4}} \right)}_3}} \right]$ is?
(A). $3$
(B). $6$
(C). $12$
(D). $2$

Answer 1

Hint: In modern technology such compounds are called coordination compounds. The chemistry of coordination compounds is an important and challenging area of modern chemistry. Variety of metallurgical processes, industrial catalysts and analytical reagents involve the use of coordination compounds.

Complete answer:
The total number of unidentate ligands ( and double the number of bidentate ligands if any ) attached to the central metal ion through coordinate bonds is called the coordination number of the metal ion .
Or in other words , coordination number may be defined as the number of atoms of the ligands which are directly bonded to the central metal atom or ion or coordination number may be defined as the number of coordinate bonds formed with the central metal atom or ion by the ligands .
Now , our aim is to calculate the coordination number of $Cr$ in ${K_3}\left[ {Cr{{\left( {{C_2}{O_4}} \right)}_3}} \right]$ .
The complex contains three bidentate ligands $({C_2}{O_4}^{2 - })$ . Didentate means the ligand contains two donor atoms .
Hence the coordination number of $Cr$ in ${K_3}\left[ {Cr{{\left( {{C_2}{O_4}} \right)}_3}} \right]$ is 6 .
Hence , option B is correct .

Note:
It is a very important point that coordination number of the central metal atom or ion is determined by the number of sigma bonds formed by the ligands with the central metal atom or ion. Pi bonds , if any , formed between the ligand and the central metal atom/ion are not considered for this purpose . The electrons forming sigma bonds are sometimes represented by a pair of dots preceding the donor atom of the ligand in the formula .