The crystal field splitting energy is lesser than pairing energy( ∆_o<P) for complexes containing weak field ligands. So, pairing here is not preferred(higher energy, unstable)

The crystal field splitting energy is greater than pairing energy( ∆_o>P) for complexes containing strong field ligands. So, pairing is preferred here(lower energy, stable)

[Three of the t_2gorbitals can hold two electrons each so they can hold a total of 6 electrons.

Two of the e_g orbitals can hold two electrons each so they can hold a total of 4 electrons.]

[CoF₆]^3–

F is a weak field ligand. Pairing is not preferred. so the two electrons instead of pairing in t_2g

Co³⁺(d⁶)has electronic configuration t_2g⁴ e_g²

So, the two electrons instead of pairing in t_2g go to e_g orbitals.

[Fe(CN)₆]^4–

CN is a strong field ligand. Pairing is preferred.

Fe²⁺(d⁶) has electronic configuration t_2g⁶ e_g⁰

So, all the six electrons get paired up in t_2g and no electron enters the e_g orbitals

[Cu(NH₃)₆]²⁺

NH₃ is a strong field ligand. Pairing is preferred.

Cu²⁺ (d⁹) has electronic configuration t_2g⁶ e_g³

The six electrons first get paired up in t_2g and the remaining 3 electrons enter the e_g orbitals and they too get paired up leaving behind only one unpaired e_g electron.