Among $\left[\mathrm{Ni}{\left(\mathrm{CO}\right)}_4\right], [\mathrm{Ni}{\left(\mathrm{CN}\right)}_4{]}^{2-}$and ${\left[{\mathrm{NiBr}}_4\right]}^{2-}$ species, the hybridization state of Ni atoms are respectively:

The valence electronic configuration of Ni in $\left[\mathrm{Ni}{\left(\mathrm{CO}\right)}_4\right]$ is 3d⁸4s². As CO is a strong ligand, therefore pairing of Ni electrons takes place and the complex has sp³ hybridization.

The valence electronic configuration of Ni in ${\left[\mathrm{Ni}{\left(\mathrm{CN}\right)}_4\right]}^{2-}$is 3d⁸. As CN is a strong ligand, therefore pairing takes place and the complex opts dsp² hybridization.

The valence electronic configuration of Ni in $[\mathrm{Ni}{\left(\mathrm{Br}\right)}_4{]}^{2-}$ is 3d⁸4s². As Br is a weak ligand, therefore the electrons of Ni remained unpaired and the complex shows sp³ hybridization.