Coordination Compounds
Coordination compounds are formed when a central metal atom/ion binds a fixed number of ions or neutral molecules (ligands) by sharing electron pairs. They are central to inorganic, bio-inorganic and industrial chemistry (chlorophyll, haemoglobin, vitamin B12).
Werner's theory
- Metals show two valences: primary (ionisable, satisfied by negative ions) and secondary (non-ionisable, satisfied by neutral molecules or anions).
- Secondary valence = coordination number, fixed for a metal, with definite spatial arrangement (octahedral, square planar, tetrahedral).
- Species in the square bracket = coordination entity; ions outside = counter ions. Complexes (e.g. [Fe(CN)6]4-) do not dissociate into simple ions, unlike double salts.
Key terms
- Ligand denticity: unidentate (Cl-, NH3, H2O), didentate (en, oxalate), polydentate, hexadentate (EDTA4-). Ambidentate = two different donor atoms, either can ligate (NO2-, SCN-). Chelate = di/polydentate forming a ring (extra stability).
- Coordination number = number of sigma-bonded donor atoms (pi bonds not counted).
- Oxidation number: charge on metal if all ligands removed with their electron pairs.
- Homoleptic (one kind of donor) vs heteroleptic (more than one kind).
Nomenclature (IUPAC)
Cation named first. Ligands named alphabetically before the metal: anionic ligands end in -o (chlorido, cyanido), neutral special names = aqua, ammine, carbonyl, nitrosyl. Use di/tri/tetra, or bis/tris/tetrakis when the ligand name has a prefix. Oxidation state = Roman numeral. Anionic complex: metal name ends in -ate (ferrate, cobaltate).Isomerism
- Stereoisomerism: geometrical (cis/trans, fac/mer) and optical (chiral enantiomers d/l).
- Structural isomerism: linkage (ambidentate ligand), coordination (ligand swap between cation/anion entities), ionisation (counter ion <-> ligand) and solvate/hydrate.
Bonding theories
- VBT: metal uses (n-1)d ns np (inner, d2sp3, low spin) or ns np nd (outer, sp3d2, high spin) for hybridisation; predicts geometry and magnetism but not colour.
- CFT: electrostatic model. Octahedral field splits d into lower t2g (-2/5 delta-o) and higher eg (+3/5 delta-o). delta-o vs pairing energy P decides high vs low spin. Tetrahedral splitting is inverted and smaller (delta-t = 4/9 delta-o), so low spin is rare; no g subscript (no centre of symmetry). Colour = d-d transition; observed colour is complementary to that absorbed.
- Spectrochemical series (increasing field): I- < Br- < SCN- < Cl- < F- < OH- < oxalate < H2O < NH3 < en < CN- < CO.