IIT JEE Advanced 2006 Paper 1

In K2[Ni(CN)4] ; Ni2+(3d8)
The electronic configuration is

↿⇂

↿⇂

↿⇂

↿

↿

CN− being a strong-field ligand causes pairing of electrons

↿⇂

↿⇂

↿⇂

↿⇂

↿

So, no unpaired electrons, hence the complex is diamagnetic.
In K2[Ni(Cl)4] ; Ni2+(3d8)
The electronic configuration is

↿⇂

↿⇂

↿⇂

↿

↿

Cl− being a weak-field ligand does not cause pairing of electrons, so there are two unpaired electrons and the complex is paramagnetic.