Match List I with List II.

A-II, B-III, C-I, D-IV

In the diamagnetic octahedral complex, [Co(NH₃)₆]³⁺, the cobalt ion is in +3 oxidation state and has the electronic configuration 3d⁶.

Six pairs of electrons, one from each NH₃ molecule, occupy the six hybrid orbitals. Thus, the complex has octahedral geometry and is diamagnetic because of the absence of unpaired electron. In the formation of this complex, since the inner d orbital (3d) is used in hybridization, the complex, [Co(NH₃)₆]³⁺ is called an inner orbital or low spin or spin paired complex.

The paramagnetic octahedral complex, [CoF₆]^3– uses outer orbital (4d) in hybridization (sp³d²). It is thus called outer orbital or high spin or spin free complex.

In [NiCl₄]^2-, nickel is in +2 oxidation state and the ion has the electronic configuration 3d⁸.

Each Cl^– ion donates a pair of electrons. The compound is paramagnetic since it contains two unpaired electrons.

[Ni(CO)₄] has tetrahedral geometry but is diamagnetic since nickel is in zero oxidation state and contains no unpaired electron.