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.
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- ✓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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