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Inner/outer orbital complexes; VBT of [Fe(CN)₆]³⁻

Short answer

Inner orbital complexes use (n-1)d orbitals (e.g., d2sp3), low spin; outer orbital use nd (sp3d2), high spin. [Fe(CN)6]3-: Fe3+ with strong CN- pairs d electrons, one unpaired, d2sp3 hybridization (inner orbital), octahedral, paramagnetic (μ=1.73 BM).

What are the inner orbital and outer orbital complexes? Explain formation of [Fe(CN)₆]³⁻ on the basis of valence bond theory and predict its magnetic behavior.

Inner and outer orbital complexes

  • Inner orbital complex: the metal uses inner (n−1)d orbitals for hybridisation (d²sp³ or dsp²). Strong-field ligands cause electron pairing, so these are low spin. Example: [Fe(CN)₆]³⁻.
  • Outer orbital complex: the metal uses outer nd orbitals (sp³d²) and the d electrons stay unpaired with weak-field ligands. These are high spin. Example: [FeF₆]³⁻.

[Fe(CN)₆]³⁻

Fe³⁺ is 3d⁵. Strong-field CN⁻ forces the five d electrons to pair in two orbitals (one electron unpaired), freeing two 3d orbitals. These with 4s and three 4p give six d²sp³ hybrids.

3d: [↑↓][↑↓][↑ ][  ][  ]  4s [ ]  4p [ ][ ][ ]
                  \___ d²sp³ ___/

Geometry: octahedral, inner orbital. Magnetism: one unpaired electron, so paramagnetic, μ = 1.73 BM.

✓ Answer checked by Claude Opus 5.5 (high effort)

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