Coordination Compounds
Coordination Compounds marks come from IUPAC nomenclature, isomer counting, magnetic moment and the bonding models (VBT and CFT).
Core rules
- Nomenclature: list ligands alphabetically, prefix anionic ligands with chloro, hydroxo, oxalato, nitrito; an anionic complex takes the Latin metal name with -ate (ferrate, cuprate); oxidation state goes in Roman numerals in brackets.
- Coordination numbers: [Co(NH₃)₆]³⁺ 6, [Ni(CN)₄]²⁻ 4, [Fe(CN)₆]⁴⁻ 6.
- Isomerism: ionisation, hydrate, linkage (NO₂⁻ as nitrito-N or nitrito-O), coordination, geometric (cis/trans) and optical (for [M(en)₃]).
- VBT shapes: dsp² square planar, sp³ tetrahedral, d²sp³ inner-orbital octahedral (low spin), sp³d² outer-orbital octahedral (high spin).
- CFT: Δo splitting; strong-field ligands (CN⁻, CO, NO₂⁻) give low spin, weak-field (I⁻, Br⁻) give high spin; Δt = (4/9)Δo.
- EAN rule: metal-ion electrons + 2 × donated pairs; 36 signals a stable krypton-like count.
Worked example
Analyse [Co(NH₃)₆]Cl₃. Three chloride ions mean cobalt is +3, i.e. d⁶. NH₃ with Co³⁺ produces a low-spin t2g⁶ eg⁰ arrangement with zero unpaired electrons, so the complex is octahedral and diamagnetic. Check the electron count: (27 − 3) + 6 × 2 = 36, matching the inert-gas rule.
Traps that cost marks
- Naming ligands in order of charge instead of alphabetically.
- Omitting the -ate ending for anionic complexes.
- Assuming [NiCl₄]²⁻ is square planar - it is tetrahedral with two unpaired electrons, while [Ni(CN)₄]²⁻ is square planar and diamagnetic.
- Missing ionisation isomers such as [Co(NH₃)₅Br]SO₄ versus [Co(NH₃)₅SO₄]Br.
- Counting total electrons instead of unpaired d electrons for μ.