Magnetic properties and limits of VBT

Magnetic susceptibility measurements give the magnetic moment, hence the number of unpaired electrons and a clue to the structure.

d¹ to d³ ions (Ti³⁺, V³⁺, Cr³⁺) always have two vacant 3d orbitals for octahedral hybridisation, so the free ion and its complexes behave alike magnetically.

For d⁴ (Cr²⁺, Mn³⁺), d⁵ (Mn²⁺, Fe³⁺) and d⁶ (Fe²⁺, Co³⁺), two 3d orbitals can be emptied only by pairing, which leaves 2, 1 and 0 unpaired electrons respectively.

Magnetic data show both behaviours: [Mn(CN)₆]³⁻ has 2 unpaired electrons but [MnCl₆]³⁻ has 4; [Fe(CN)₆]³⁻ has 1 but [FeF₆]³⁻ has 5; [Co(C₂O₄)₃]³⁻ is diamagnetic but [CoF₆]³⁻ has 4.

VBT's answer: [Mn(CN)₆]³⁻, [Fe(CN)₆]³⁻ and [Co(C₂O₄)₃]³⁻ are inner orbital (d²sp³) complexes; [MnCl₆]³⁻, [FeF₆]³⁻ and [CoF₆]³⁻ are outer orbital (sp³d²) complexes.

Example: [MnBr₄]²⁻ has a spin-only moment of 5.9 BM, meaning five unpaired electrons, so Mn²⁺ has not paired any d electrons: the ion is tetrahedral (sp³), not square planar (dsp²).

Limits of VBT: it rests on several assumptions, gives no quantitative account of magnetic data or of thermodynamic and kinetic stability, says nothing about colour, cannot reliably choose between tetrahedral and square planar for CN 4, and does not separate weak from strong ligands.