Coordination Compounds
Coordination compounds tests nomenclature, isomerism, bonding models and applications. The exam often asks you to deduce geometry, magnetic behaviour and hybridisation from a formula.
Nomenclature and isomerism
Name ligands alphabetically, use prefixes di-, tri-, bis-, tris-, and indicate the oxidation state of the metal in Roman numerals. Structural isomerism includes ionisation, hydrate, linkage and coordination isomerism; stereoisomerism includes geometrical (cis/trans) and optical forms. Ambidentate ligands such as SCN− can bind through S or N.
VBT and CFT
Valence bond theory counts hybrid orbitals from ligand approach: coordination number 6 gives octahedral d2sp3 (inner orbital, low spin) or sp3d2 (outer orbital, high spin); CN 4 gives tetrahedral sp3 or square planar dsp2. Crystal field theory splits d orbitals into t2g and eg by Δo; strong-field ligands give low spin, weak-field give high spin. Δo increases along I− < Br− < Cl− < F− < OH− < H2O < NH3 < en < CN− < CO.
Worked example
[Co(NH3)6]3+: Co3+ is d6. NH3 is a strong-field ligand, so electrons pair in t2g, giving t2g6 eg0, no unpaired electrons, diamagnetic, low-spin octahedral. [CoF6]3− with weak-field F− is high spin with four unpaired electrons. The reasoning is: find metal oxidation state, count d electrons, place ligands in the spectrochemical series, then fill t2g/eg.
Mark-losing traps
Confusing charge on the complex with oxidation state, using the wrong prefix for a polydentate ligand, assuming every four-coordinate complex is tetrahedral, and forgetting that [Ni(CN)4]2− is square planar while [NiCl4]2− is tetrahedral are common errors.