The I.U.P.A.C. name for [Ni(NH3)4][NiCl4] is:
Tetraamminenickel (II) - tetrachloridonickelate (II)
We name the cation before the anion.
The given compound is made of [Ni(NH3)4]2+ and [NiCl4]2−.
The cation will then be named as Tetraamminenickel(II) while the cation is named as
tetrachloridonickelate(II). The ending 'ate' signifies that this species is in fact the anion.
Thus the IUPAC name is: Tetraamminenickel(II) tetrachloridonickelate(II)
Alternatively, the name could also be: Tetraamminenickel(2+) tetrachloridonickelate(2−).