In [Ni(CN)₄]^2-, nickel is in +2 oxidation state and has the electronic configuration 3d⁸ 4s⁰

As CN- is a strong field ligand, pairing of electrons takes place and resulting in empty one 3d orbital, one 4s orbital and two 4p orbitals which undergoes hybridisation to form four dsp²-hybrid orbitals. These hybrid orbitals occupied by electron pairs of four CN- ions.
∴ [Ni(CN)₄]^2-=

Thus, the hybridisation involved is dsp² and therefore, the species is square planar and diamagnetic as all electrons are paired.

lUPAC name is: Hexafluorocobaltate (III) ion

When the complex formed involves the inner (n -1) d-orbitals for hybridization (d²sp³), the/complex is called inner orbital complex. [Co(NH₃)₆]³⁺ involves inner (n - 1) d-orbitals. Therefore, it is called inner orbitals complex. When the complex formed involves the use of outer n d-orbitals for hybridization (sp³d²), the complex is called outer orbital complex. [Ni(NH₃)₆]²⁺ involves (n - 1) d-orbitals. Therefore, it is outer complex.
[Co(NH₃)₆]³⁺ Co = 27 = 4s²3d⁷
Co³⁺ = 3d⁶

(i) Pentaamminechloridocobalt (Ill) ion
(ii) K₂[NiCI₄]

Tetraamminedichlorochromium (III). It exhibits geometrical isomerism.

Cr(24) has outer electronic configuration 4s¹3d⁵.
Cr³⁺(24):4s⁰3d³.
H₂O is weak field ligand, it cannot cause pairing of electrons.

Linkage isomerism.

Out of the two, (a) cis - [CrCl₂(ox)²]^3- is chiral (optically active).