Coordination Compounds
What this domain tests
Werner's theory, IUPAC nomenclature, isomerism, valence bond and crystal field interpretations, magnetic behaviour, stability and applications.
Core rules
- Naming: cite ligands alphabetically, ignoring di-, tri- and tetra- as ordering prefixes but keeping bis, tris and tetrakis for ligands that already contain a numerical prefix. Then name the metal with its oxidation state in Roman numerals. An anionic complex ends in -ate.
- Isomerism: ionisation, hydrate, linkage (NO₂⁻ versus ONO⁻), coordination, geometrical (cis/trans, fac/mer) and optical.
- VBT: d²sp³ is inner-orbital (low spin), sp³d² is outer-orbital (high spin), sp³ is tetrahedral and dsp² is square planar.
- CFT: Δo separates t2g and eg. Strong-field ligands (CN⁻, CO, NO₂⁻) cause pairing; weak-field ligands (I⁻, Br⁻, Cl⁻, F⁻, H₂O) favour high spin. CFSE and spectrochemical series predict colour and magnetism.
- Applications: EDTA as a hexadentate chelating titrant, cisplatin (only the cis isomer is active), chlorophyll (Mg), haemoglobin (Fe) and vitamin B₁₂ (Co).
Worked example
In [Co(NH₃)₆]³⁺, let cobalt be x with six neutral NH₃ ligands: x + 0 = +3, so Co is +3, a d⁶ ion. NH₃ produces sufficient splitting for Co(III) to adopt a low-spin t2g⁶ configuration. Hybridisation is d²sp³, the geometry is octahedral, and there are no unpaired electrons, so the complex is diamagnetic with μ = 0 BM.
Traps that cost marks
Always calculate the metal oxidation state from the overall charge. Alphabetical ordering can change the name completely. Do not confuse [Ni(CN)₄]²⁻ (square planar, dsp², diamagnetic) with [NiCl₄]²⁻ (tetrahedral, sp³, paramagnetic). Tetrahedral complexes are generally high spin because Δt is small.