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Among the coordination compounds listed below, which one is an outer orbital complex (high-spin, uses outer d orbitals for hybridisation)?

1. [Fe(CN)6]4-
2. [Mn(CN)6]4-
3. [Cr(NH3)6]3+
4. [Ni(NH3)6]2+

Correct answer: [Ni(NH3)6]2+

Solution

[Ni(NH3)6]2+ has Ni2+ in d8 configuration; NH3 is not strong enough to cause pairing in 3d, so hybridisation involves outer 4s4p4d orbitals (sp3d2), making it an outer orbital (high-spin) complex.

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