The number of unpaired electrons in $Ni(CO)_4$ is ..........

Which of the following statements is correct regarding the complexes $[CoF_6]^{3-}$ and $[Co(NH_3)_6]^{3+}$?

$[NiCl_{4}]^{2-}$ is paramagnetic while $[Ni(CO)_{4}]$ is diamagnetic though both are tetrahedral. Why?

The structure of iron pentacarbonyl is:

Both $[Ni(CO)_4]$ and $[Ni(CN)_4]^{2-}$ are diamagnetic. The types of hybridisation of $Ni$ in these complexes are . . . . . . and . . . . . . respectively.

Which complex has a square planar structure?