Coordination Compounds
Domain 9: Coordination Compounds
This domain tests nomenclature, isomerism, bonding models, magnetic behaviour, and the stability and applications of complexes.
Core rules
- Coordination number is the number of donor atoms bonded to the central metal, not the number of ligands (ethylenediamine is bidentate).
- Nomenclature: name ligands alphabetically, use prefixes, then metal with oxidation state in Roman numerals; an anionic complex ends in -ate. Example: [Co(NH3)6]Cl3 is hexaamminecobalt(III) chloride.
- Isomerism includes geometrical (cis/trans in square planar and octahedral), optical, ionisation, hydrate, linkage, and coordination isomerism.
- Crystal field theory: ligands split d orbitals into t2g and eg. Strong-field ligands (CN−, CO) give low spin and large Δo; weak-field (I−, Br−) give high spin. Colour arises from d–d absorption; magnetic moment from unpaired electrons.
- Stability is thermodynamic (Kf); chelating ligands such as EDTA increase stability by entropy.
Worked example
[Fe(CN)6]3−: let Fe oxidation state be x; x+6(−1)=−3, so x=+3, Fe3+ is d5. CN− is strong field, so low-spin t2g5 eg0: one unpaired electron. μ=√(1 x 3)=1.73 BM; the complex is paramagnetic. By contrast, [Fe(CN)6]4− has Fe2+ d6 low spin and is diamagnetic.
Traps
- Counting ligands instead of donor atoms.
- Getting the metal oxidation state wrong when the complex carries a charge.
- Assuming tetrahedral complexes are always low spin; tetrahedral splitting is small.
- Confusing coordination isomerism with ionisation isomerism.
- Forgetting that optical isomerism requires non-superimposable mirror images.