Paramagnetic substances have one or more unpaired electrons and are weakly attracted to a magnetic field. Diamagnetic substances have all electrons paired and are weakly repelled by a magnetic field.
Strong field vs weak field ligands (spectrochemical series, partial list):
I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < ox²⁻ < H₂O < NH₃ < en < CN⁻ < CO
High spin vs low spin for d⁶ (Co³⁺, Fe²⁺): With weak field ligands (F⁻, ox²⁻): high spin, 4 unpaired electrons. With strong field ligands (CN⁻): low spin, 0 unpaired electrons. The crossover depends on whether CFSE exceeds pairing energy.
Geometry rules: Strong field + Ni²⁺ (d⁸) → square planar (Ni(CN)₄²⁻) → diamagnetic. Weak field + Ni²⁺ → tetrahedral (NiCl₄²⁻) → paramagnetic (2 unpaired). Ni(CO)₄ is Ni⁰ with d¹⁰ → always diamagnetic.
Key d-electron counts: d⁰ and d¹⁰ are always diamagnetic (no unpaired) or always have n unpaired respectively. d¹⁰ = 0 unpaired. d⁵ high spin = 5 unpaired (maximum).
Ligands ordered by field strength: I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < ox < H₂O < NH₃ < en < CN⁻ < CO. Weak field → high spin (more unpaired). Strong field → low spin (fewer unpaired). Memorising this series is essential.
For d⁶ metal with weak ligand: 4 unpaired (high spin). With strong ligand: 0 unpaired (low spin). For d⁸ Ni²⁺: tetrahedral (weak) = 2 unpaired; square planar (strong) = 0 unpaired. For d¹⁰ (Ni⁰ in Ni(CO)₄): 0 unpaired, always diamagnetic.
Oxalate (C₂O₄²⁻) is a moderate field ligand — weaker than CN⁻ but stronger than halides. Co³⁺ (d⁶) with oxalate is high spin → 4 unpaired → paramagnetic. Compare: [Co(CN)₆]³⁻ would be low spin → 0 unpaired.
Low spin Mn³⁺ d⁴: electrons fill t₂g as: t₂g⁴ eg⁰. With strong CN⁻, pairing occurs in t₂g → 2 unpaired electrons (t₂g has 3 orbitals, 4 electrons → one orbital has 2, two orbitals have 1 each = 2 unpaired). Paramagnetic.