Ligand donor atoms surround a central metal in a coordination sphere.
Key knowledge
Coordination chemistry describes central atoms or ions, commonly metals, bound to ligands. The coordination number counts attached donor atoms, while ligand denticity counts donor atoms supplied by one ligand. Geometry and ligand arrangement can produce distinct isomers.
Conditions and scope
Coordination number need not equal the number of ligand molecules.
Electron-pair donation is introductory Lewis bookkeeping; it does not fully describe every metal–ligand interaction.
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Coordination chemistry describes central atoms or ions, commonly metals, bound to ligands. The coordination number counts attached donor atoms, while ligand denticity counts donor atoms supplied by one ligand. Geometry and ligand arrangement can produce distinct isomers.
Ligand arrangements can produce geometric and optical isomers.
A Lewis base donates an electron pair and a Lewis acid accepts it in forming an adduct. A metal–ligand formation constant describes a specified equilibrium; stepwise formation constants multiply to give the corresponding cumulative constant. Free ligand and metal activities, rather than analytical totals, enter that equilibrium quotient.
In an octahedral crystal-field model, d orbitals separate into a lower t2g set and an upper eg set. For eligible electron counts, the competition between splitting and electron-pairing energy produces different spin occupations. Geometry and ligand interactions affect splitting; crystal-field electrostatics is an approximation to metal–ligand electronic structure.
In a common mononuclear two-electron oxidative addition, a substrate bond is replaced by two metal–ligand bonds and the metal's formal oxidation state increases by two. Reductive elimination reverses that accounting by forming a ligand–ligand bond. These transformations connect steps in catalytic cycles, with several possible detailed mechanisms.
Ligand and formal oxidation-state accounting describe the transformation.
Coordination compounds can share composition while differing in ligand connectivity or spatial arrangement. Cis/trans isomers differ in relative ligand positions, while chiral coordination arrangements can have nonsuperimposable mirror images. Octahedral tris-chelate arrangements provide an example of chirality without requiring a tetrahedral carbon stereocentre.