VBT formation of [Ni(CO)₄] and [Fe(CN)₆]⁴⁻

[Ni(CO)₄]⁰

Ni (0) is 3d⁸ 4s². CO is a strong-field ligand, so the 4s electrons move into 3d, giving 3d¹⁰. The vacant 4s and three 4p orbitals form four sp³ hybrid orbitals, which accept four CO lone pairs.

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

Shape: tetrahedral; no unpaired electron, so diamagnetic.

[Fe(CN)₆]⁴⁻

Fe²⁺ is 3d⁶. CN⁻ is a strong-field ligand, so the six electrons pair in three 3d orbitals and two 3d orbitals are vacant. Two 3d, one 4s and three 4p orbitals form six d²sp³ hybrids that accept six CN⁻ lone pairs.

3d: [↑↓][↑↓][↑↓][  ][  ]  4s [ ]  4p [ ][ ][ ] → d²sp³

Shape: octahedral (inner orbital); no unpaired electron, so diamagnetic.