Given are the nickel compounds $\mathrm{Ni}\left({\mathrm{CO}}_4\right), {\mathrm{K}}_2\left[{\mathrm{NiF}}_6\right]$ and ${\mathrm{K}}_2\left[\mathrm{Ni}(\mathrm{CN}{)}_4\right]$ The order of compounds having increasing oxidation number of nickel is

The oxidation number of nickel in $\mathrm{Ni}(\mathrm{CO}{)}_4, {\mathrm{K}}_2\left[\mathrm{Ni}(\mathrm{CN}{)}_4\right]$ and ${\mathrm{K}}_2\left[{\mathrm{NiF}}_6\right]$ are 0, +2 and +4, respectively.

The total number of electrons that an atom acquires or loses in the process of joining with another atom to create a chemical bond is known as the oxidation number, also known as the oxidation state.