(a) (i) [FeF6]3-
• The electronic configuration of Fe (26) is [Ar] 3d64s2. The oxidation state of Fe in [FeF6]3- is +3.
• This leads to the electronic configuration of Fe3+ becoming [Ar] 3d54s0.
• The electrons from the fluorine atoms now overlap with the vacant orbitals of Fe ion.
• Since there are 6 F atoms, 6 orbitals are required. So, one 4s orbital, three 4p orbitals and two 4d orbitals undergo sp3d2 hybridisation to form 6 orbitals of same energy.
• Now, since fluorine is a strong ligand, pairing does not take place.
• Thus, the complex [FeF6]3- is paramagnetic and has octahedral geometry.
(ii) [Ni (CO)4]
• The electronic configuration of Ni (28) is [Ar] 3d84s2. The oxidation state of Ni in [Ni (CO)4] is 0.
• Now, since 4 orbitals are required for the overlapping of the electrons of carbonyl group, the two electrons from the 4s orbital shift to the 3d orbital.
• The configuration now becomes [Ar] 3d104s0.
• One 4s and three 4p orbitals undergo sp3 hybridisation to form 4 orbitals of equal energy.
• The electrons from CO group now overlap with the orbitals to form [Ni (CO)4] complex.
• Thus, [Ni (CO)4] is diamagnetic and tetrahedral.
• On moving across the spectrochemical series we see that CO is a stronger ligand than CN-.
• Thus, complexes formed with CO group tend to be more stable than those formed by CN- ligand.
• Another reason is that CO is capable of forming synergic bonds which literally translated to ‘self-strengthening’ bond.
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