Question

The coordination number of Co in $\left[ {Co{{(en)}_2}C{l_2}} \right]Cl$ is:
$
  (a){\text{ 3}} \\
  (b){\text{ 4}} \\
  (c){\text{ 5}} \\
  (d){\text{ 6}} \\
 $

Answer 1

Hint – In this question use the concept that (en) is a bi-denate ligand and Cl is a mono-denate ligand, simply bind Co with two (en) ligand and two Cl ligand. Application of this with the basic definition of coordination number will help to get the answer.

Complete answer:
As we know that the coordination number is defined as the number of coordinate bonds (i.e. in the coordination sphere) of the central metal atom (Co) with the ligands (en) and (Cl).
Now as we know (en) is a bi – dentate ligand and Cl is a mono – dentate ligand.
So in the given chemical formula Co binds with the two (en) ligand and two (Cl) ligand.
So the coordination number of Co is = $\left[ {2 \times 2 + 2 \times 1} \right]$
Therefore coordination number of Co is $ = 4 + 2 = 6$.
So this is the required answer.

Hence option (D) is the correct answer.

Note – Ligand is considered as an ion or molecule attached to a metal atom by coordinate bond. The trick point here is that we have considered Co as the main central atom, this is because the central atom is the one with the lowest electronegativity, and electronegativity can easily be compared by the view of periodic table.