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Question
Explain [Co(NH3)6]3+ is an inner orbital complex, whereas [Ni(NH3)6]2+ is an outer orbital complex.
[At. No.: Co = 27, Ni = 28]
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Solution
When a complex uses the inner (n − 1)d orbitals for hybridisation to form d2sp3 hybrid orbitals, it is known as an inner-orbital complex. The complex [Co(NH3)6]3+ uses inner (n − 1)d orbitals and is therefore classified as an inner orbital complex.
On the other hand, when a complex is formed using the outer d orbitals for hybridisation, resulting in 2sp3d2 hybrid orbitals, it is called an outer orbital complex. The complex [Ni(NH3)6]2+ uses the outer orbitals and is therefore an outer orbital complex.
[Co(NH3)6]3+ Co = 27 = 4s2 3d7
Ammonia, being a strong ligand, pairs.

[Ni(NH3)6]2+ Ni = 28 = 4s2 3d8
Although it is a strong ligand, if pairing of electrons occurs, only one d-orbital remains available, so the complex cannot undergo d2sp3 hybridisation.

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