Coordination Chemistry
Coordination Chemistry tests nomenclature, isomerism, bonding models (VBT and CFT), magnetic behaviour and stability. The exam frequently combines a formula with naming, oxidation state and spin state.
Naming and structure
- Coordination number is the number of ligand donor atoms bonded to the central metal.
- Name ligands alphabetically before the metal; use aqua, ammine, carbonyl, nitrosyl, hydroxo, chloro.
- Anionic complexes end in -ate: [Fe(CN)6]4- is hexacyanoferrate(II).
- Oxidation state: sum of ligand charges plus metal charge equals complex charge.
Bonding and isomerism
- VBT: inner-orbital complexes use (n-1)d orbitals; outer-orbital use nd.
- CFT: delta-o splitting; strong-field ligands (CN-, CO) give low spin; weak-field (F-, H2O) give high spin.
- Isomerism includes geometrical, optical, ionisation, hydrate, linkage and coordination isomerism.
Worked example
Describe [Co(NH3)6]3+ in terms of spin and magnetism. Reasoning: Co3+ is d6. NH3 is a relatively strong-field ligand, so the complex is low spin: t2g6 eg0. It has no unpaired electrons, is diamagnetic, and is inner-orbital (d2sp3). By contrast, [CoF6]3- is high spin with four unpaired electrons and is paramagnetic.
Traps
- Ligand names are alphabetical, but the metal name comes after all ligands.
- Do not confuse charge of the complex with oxidation state of the metal.
- A d6 low-spin complex is diamagnetic; a d6 high-spin complex has four unpaired electrons.
- Linkage isomerism requires an ambidentate ligand such as SCN- or NO2-.