Question

In the complexes $\left[Fe\right(CN{)}_6{]}^{3-}\text{and}\left[Co\right(en{)}_3{]}^{3+}$, the coordination number of iron and cobalt are respectively (en = ethylenediamine):

• A. $6\text{and}6$
• B. $12\text{and}6$
• C. $6\text{and}3$
• D. $3\text{and}3$

Answer 1

The correct option is A $6\text{and}6$

In $1^{st}$ compound. $CN$ is a unidentate ligand and hence will surround the $Fe$ atom from $6$ different sides.
On the other hand, the $2^{nd}$ compound contains bidentate ligand so each $\left(en\right)$ will have two different sites of donating electrons and hence it required $3\text{(en)}$ atoms for surrounding $Co$ atom from $6$ different sides.
Hence (a) is correct.