Question

Using the valence bond approach, explain the shape and magnetic behaviour of [Ni(NH₃)₆]²⁺.

Answer 1

Oxidation State of Nickel:

• In the complex [Ni(NH₃)₆]²⁺ the ammonia (NH₃) ligands are neutral, so the charge on the nickel ion is +2.
• Therefore, Nickel (Ni) is in the +2 oxidation state, i.e., Ni²⁺.

Electron Configuration of Ni²⁺:

• Nickel has an atomic number of 28, so the electron configuration of neutral Ni is:
  Ni : [Ar] 3d⁸ 4s²
• For Ni²⁺, it loses two electrons (the two 4s electrons):
  Ni²⁺ : [Ar] 3d⁸
• The electron configuration of Ni²⁺ is 3d⁸.

Hybridisation and Geometry:

• In [Ni(NH₃)₆]²⁺ the coordination number of the nickel ion is 6 because it is surrounded by six NH₃ ligands.
• NH₃ is a strong-field ligand, and for a coordination number of 6, the hybridisation of the central ion is d²sp³.
• The d²sp³ hybridisation involves the mixing of two d-orbitals, one s-orbital, and three p-orbitals from the nickel ion to form six hybrid orbitals.
• These six hybrid orbitals are arranged symmetrically in an octahedral geometry.
• Therefore, the shape of the complex is octahedral.

Magnetic Behavior:

• In an octahedral field, the five degenerate ddd-orbitals split into two sets:
  The lower-energy set of t_2g​ orbitals (three orbitals).
  The higher-energy set of e_gorbitals (two orbitals).
• The NH₃ ligands are strong-field ligands, meaning they cause pairing of electrons in the lower-energy t_2g​ orbitals.
• Since the Ni²⁺ ion has 8 electrons in its 3d orbitals (3d⁸ configuration), the strong-field effect of NH₃ forces all electrons to pair up in the lower-energy t_2g​ orbitals.
• As a result, there are no unpaired electrons in the complex, and it is diamagnetic.