Question
(JEE Main pattern) For the complex , find: (a) oxidation state of cobalt, (b) coordination number, (c) hybridisation of central atom, (d) magnetic behaviour, (e) IUPAC name.
Solution — Step by Step
NH₃ is neutral. Charge on complex = +3. So oxidation state of Co = +3.
Six NH₃ ligands attached, each contributing one lone pair (monodentate). Coordination number = 6.
Co³⁺ has electron configuration . NH₃ is a strong-field ligand → causes pairing of the d electrons.
In strong field with 6 d electrons all paired in three orbitals, the two orbitals are empty. The two empty 3d orbitals, plus 4s and three 4p, form d²sp³ hybrid orbitals (inner orbital complex).
All electrons paired → diamagnetic.
Hexa- (6 ligands) + ammine (NH₃) + cobalt + (III) for oxidation state. The complex is a cation, so we give the metal’s name first.
Hexaamminecobalt(III) ion. With chloride counterion: hexaamminecobalt(III) chloride.
Summary: (a) +3, (b) 6, (c) d²sp³, (d) diamagnetic, (e) hexaamminecobalt(III).
Why This Works
Coordination compound questions follow a fixed recipe:
- Charge balance → oxidation state.
- Count ligands → coordination number.
- Use the spectrochemical series to decide strong/weak field → pairing → hybridisation.
- Count unpaired electrons → magnetic moment BM.
- Name with prefixes (mono, di, tri, hexa) and ligand-then-metal order.
NH₃, CN⁻, CO are strong-field ligands. F⁻, Cl⁻, H₂O are weak-field. Memorize this — JEE Main asks every year.
Inner orbital complex (d²sp³) → low spin → fewer unpaired electrons → diamagnetic or weakly paramagnetic. Outer orbital complex (sp³d²) → high spin → more unpaired → strongly paramagnetic.
Alternative Method
Crystal field theory: in octahedral strong field, (pairing energy), so d⁶ pairs in — low spin, diamagnetic. Same conclusion via CFT instead of VBT (valence bond theory).
Common Mistake
Students name the complex as “cobalt(III) hexaammine” — wrong order. In IUPAC nomenclature, ligands come first (alphabetical, with prefixes ignored for ordering), then metal with oxidation state. So “hexaamminecobalt(III)”, not the reverse.