[Ni(Cl)4]2- is paramagnetic while [Ni(Co)4]is diamagnetic though both are tetrahedral. Why?

In [Ni(Cl)4]2- ion, Cl- is a weak field ligand so it will not pair the unpaired electrons of Ni +2 ion. 

Electronic configuration of Ni is: [Ar]3d84s2 where [Ar] = 1s22s22p63s23p6

Electronic configuration of Ni + 2 = [Ar]3d8

Outer electronic configuration of Ni + 2 = 3d8

Overall charge balance:

X + 4(-1) = -2

X = + 2.

Therefore it undergoes sp3 hybridization. So it will have tetrahedral geometry.

Since there are 2 unpaired electrons in the d orbital so it is a paramagnetic compound.

In [Ni(Co)4]:

Overall charge is neutral and oxidation state of Ni can be calculated as:

X + 4(0) = 0

X = 0

Ni is in zero oxidation state.

Co is a strong field ligand it causes pairing of the 4 unpaired electrons in d orbital. Also,it causes the 4s electrons to shift to the 3d orbital, thereby undergoes sp3 hybridisation forming tetrahedral geometry. Since it has no unpaired electron , therefore it is diamagnetic compound.