Read How Ligand Strength Decides Hybridisation
Valence bond theory explains geometry and magnetism through which orbitals hybridise — strong-field ligands force pairing and use inner (n-1)d orbitals; weak-field ligands don't, and use outer nd orbitals instead.
Electricity questions depend on identifying charge, a conducting path and the condition that controls current or magnetic effect.
Same metal, different ligand, different geometry
Nickel reacts differently depending on the ligand: Cl⁻ gives a tetrahedral complex (sp₃, weak field, no pairing forced), CN⁻ gives a square planar complex (dsp₂, strong field, forces pairing), and CO gives a tetrahedral carbonyl (sp₃) that only forms in reducing conditions. Reading the ligand's field strength tells you the hybridisation and shape before you calculate anything. Choose A.
Strong-field ligands force inner-orbital (d₂sp₃) complexes
K4[Fe(CN)₆]: CN⁻ is strong-field, forcing all Fe²⁺ (d₆) electrons to pair into t₂g, freeing up two (n-1)d orbitals for d₂sp₃ hybridisation — octahedral, diamagnetic. Compare [Cu(NH₃)₄]2+: Cu²⁺ is d₉, and no amount of ligand strength can pair away a lone 9th electron, so it stays paramagnetic regardless — hybridisation (dsp₂, square planar) is a separate question from magnetism here.
Weak-field ligands keep complexes paramagnetic via sp₃d₂
K3[CoF₆] is a high-spin complex: F⁻ is weak-field, so Co³⁺ (d₆) keeps 4 unpaired electrons and uses outer-orbital sp₃d₂ hybridisation rather than pairing into d₂sp₃.
for the same octahedral geometry.
One geometry can have more than one valid hybridisation description
Some geometries are ambiguous by hybridisation scheme alone — certain d₈ square planar systems admit more than one textbook-valid hybridisation assignment for the identical observed shape, which is exactly the kind of 'which geometry allows more than one hybridisation description' trap this topic sets.
Metal carbonyls: Fe(CO)₅ is dsp₃ and uses the dz₂ orbital
In Fe(CO)₅ the CO ligands are strong-field π-acceptors, so the eight valence electrons of Fe(0) (3d₆ 4s₂) rearrange into 3d₈ with the 4s orbital emptied. Four of the five d orbitals (dxy, dxz, dyz and dx₂-y₂) hold these eight electrons, leaving dz₂ empty. That empty dz₂ orbital joins the 4s and the three 4p orbitals to make five dsp₃ hybrid orbitals, one for each CO lone pair, giving a trigonal bipyramidal, diamagnetic molecule. The orbital to remember for a trigonal bipyramid is dz₂; dx₂-y₂ is the one used in square-planar dsp₂ (and, together with dz₂, in octahedral d₂sp₃), and 'any d orbital' is never the answer because the geometry fixes which d orbital overlaps.
OlympIQ exam lens
- ✓Strong-field ligands (CN⁻, CO) force electron pairing → inner-orbital (d₂sp₃) hybridisation, usually diamagnetic.
- ✓Weak-field ligands (F⁻, H₂O) don't force pairing → outer-orbital (sp₃d₂) hybridisation, usually paramagnetic, for the same coordination number.
- ✓A d₉ ion like Cu²⁺ stays paramagnetic (1 unpaired e-) regardless of ligand strength — pairing can't remove a lone odd electron.
- ✓The same metal can show different geometries (tetrahedral, square planar) with different ligands — hybridisation follows the ligand, not just the metal.
- ✓Fe(CO)₅: Fe(0) becomes d₈ once CO pairs the electrons, hybridisation is dsp₃ using the dz₂ orbital, shape is trigonal bipyramidal, and the molecule is diamagnetic
Don't assign hybridisation from the metal alone — the SAME metal ion gives different hybridisation, geometry and magnetism with different ligands. Always check ligand field strength (spectrochemical series) before deciding pairing.
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