Tetrahedral and square planar complexes; [Ni(NH₃)₄]²⁺ VBT

Reading: the question is "What are tetrahedral and square planar complexes? Explain the formation of [Ni(NH₃)₄]²⁺ on the basis of valence bond theory and predict its magnetic behaviour."

Tetrahedral and square planar complexes

Both have coordination number 4.

• Tetrahedral: the metal uses sp³ hybrid orbitals, with the ligands at the corners of a tetrahedron (bond angle 109.5°). With weak-field ligands these complexes usually have unpaired electrons. Examples: [NiCl₄]²⁻, [Ni(CO)₄].
• Square planar: the metal uses dsp² hybrid orbitals, with the ligands at the corners of a square (bond angle 90°). This shape is common for d⁸ ions with strong-field ligands. Examples: [Ni(CN)₄]²⁻, [PtCl₄]²⁻.

[Ni(NH₃)₄]²⁺

Ni²⁺ is 3d⁸. Here NH₃ is treated as a ligand that does not force pairing of the 3d electrons, so the two unpaired electrons remain. One 4s and three 4p orbitals form four sp³ hybrids that accept the NH₃ lone pairs.

3d: [↑↓][↑↓][↑↓][↑ ][↑ ]  4s [ ]  4p [ ][ ][ ] → sp³

• Geometry: tetrahedral.
• Magnetism: two unpaired electrons, so paramagnetic, μ = 2.83 BM.