$\left[ Ni ( CO )_4\right]$ - has a tetrahedral structure.
$\left[ Ni ( Cl )_4\right]^{2-}$ - has a tetrahedral structure.
$\left[ Ni ( CN )_4\right]^{2-}$ - has a square planar structure.
In $\left[ Ni ( CN )_4\right]^{2-}, Ni$ is in $+2$ state
$
Ni ^{2+} \Rightarrow[ Ar ] 4 s ^0 3 d ^8 \Rightarrow 4 s ^0 3 d _{ xy }^2 3 d _{y z}^2 3 d _{z z }^2 3 d _{z^2}^2 3 d _{ x ^2- y ^2}^0 [\because
$
The ligand is strong it will pair up single electrons in the $d$-orbital]
Hence the hybridization would be $dsp ^2$ and the structure is square planar structure.