Ni is in zero oxidation state in [Ni(CO)4]. So, the electronic configuration of Ni is 3d84s2. As CO is a strong ligand, it pushes all the electrons in the 3d orbital, therefore the hybridisation of [Ni(CO)4]_{ }is sp3 and it has tetrahedral geometry. It is diamagnetic due to the absence of unpaired electrons.
[Cu(NH3)4]2+ ion, oxidation state of Cu is +2 and its valence shell electronic configuration is 3d9. So, there would be a rearrangement of electrons in Cu2+ because of the NH3 strong field ligand . And the last electron in the d-orbital would be out waiting for the N's electrons to fill up first. So, the 4 electron pairs from N would be in one 3d, one 4s, & two 4p orbitals and in the third place of 4p orbital, the e− from 3d would take place. So, you can say the hybridisation here would be dsp2.
[Fe(NH3)6]2+ ion, oxidation state of Fe is +2 and its valence shell electronic configuration is 3d6. NH3 is strong field ligand with complex. So, no pairing will be there. So, you can say the hybridisation here would be d2sp3.
[Fe(H2O)6]2+ ion, oxidation state of Fe is +2 and its valence shell electronic configuration is 3d6. There are 4 unpaired electrons in 3d orbital. So, you can say the hybridisation here would be sp3d2.
So, correct order of matching is given in the below table:
| List I (Complexes) | List II (Hybridisation) |
| [Ni(CO)4] | sp3 |
| [Cu(NH3)4]2+ | dsp2 |
| [Fe(NH3)6]2+ | d2sp3 |
| [Fe(H2O)6]2+ | sp3d2 |