Question

The oxidation number of cobalt in $K[Co{{(CO)}_{4}}]$ is:
A. +1
B. +3
C. -1
D. -3

Answer 1

Calculate the total charge present on the coordination complex and then consider the charge present on the ligands. Both of these together will give you the oxidation number of the complex.

Complete step by step answer:
To find the oxidation number of the central metal atom in a coordination complex, we need to know the charge present on the ligands attached to the central atom and also the overall charge of the coordination complex. For this, the fixed oxidation numbers of certain elements must be known.
In this example, the overall charge on the compound is 0. So, the charges on the metal atom i.e. potassium atom, outside the coordination complex and the coordination complex will be equal and opposite. We know that potassium has a fixed oxidation state of +1. Thus, the net charge on the coordination complex will be -1. These charges will neutralize each other and form a neutral compound. So, the molecular formula of the complex will be ${{[Co{{(CO)}_{4}}]}^{-}}$.

Now, the charge present on the coordination complex will be equal to the sum of the charges present on the metal ion as well as the ligands. We know that the carbon monoxide ligand is a neutral ligand and does not possess any charge. Thus, it will not contribute to the net charge present on the complex. So, the charge present on the complex will be equal to the charge on cobalt. Thus, the charge on cobalt in the compound $K[Co{{(CO)}_{4}}]$ is -1.
So, the correct answer is “Option C”.

Note: Note that metals usually do not have negative oxidation states since they have many free electrons that can be donated. Do not be confused since a metal has a negative oxidation state here, this may occur due to the presence of a more electropositive compound.