A ligand field describes how ligands split and mix the central metal orbitals. Sigma donation, pi donation and pi backbonding explain trends that a purely electrostatic picture misses.

Electron counting records metal valence electrons plus ligand donations using a declared convention. In ionic counting, subtract metal oxidation state from its group number, then add donor electrons; in neutral counting, combine neutral metal and ligand fragments consistently. An Fe–Fe single bond contributes one electron to the count at each metal.

The trans effect is a kinetic influence whereby a ligand accelerates substitution opposite itself, classically in square-planar complexes. It must be distinguished from ground-state bond weakening and from the carbonyl cis effect.

Chelation binds multiple donor sites of one ligand to the same central atom, forming a chelate ring. It can retain a donor near a metal even when one attachment temporarily opens.

A metal cluster contains several connected metal centers. A triangular three-metal cluster can support three edge metal-metal bonds; early-metal clusters often have stable counts below the 18-electron rule.

The Dewar-Chatt-Duncanson model describes an alkene–metal bond through donation from the filled alkene $\pi$ orbital to the metal and pi backbonding from the metal into alkene ${\pi }^{\ast }$. Backbonding weakens and lengthens the alkene double bond. A $d^0$ early-metal ion has little conventional $d$-electron backbonding.

The square-planar coordination compound cis-diamminedichloroplatinum(II), with Pt(II), a $d^8$ metal configuration and a 16-electron local valence count. This article concerns its coordination chemistry.

An octahedral platinum(IV) complex containing two chloride, two acetate and two neutral amine donors; its $d^6$ metal configuration is low spin. This article concerns its coordination chemistry.

The presence of distinguishable formal oxidation states of the same element in a compound. Formal localization need not imply complete physical localization of charge.

Different distributions of water between the metal coordination sphere and lattice for the same overall composition. Geometrical isomers can occur within one hydration class.

Replacement of one coordinated ligand by another. Formal metal oxidation state need not change, even though local electron count can change while association or dissociation occurs.

A ligand for which assigning an integer oxidation state separately from the metal can obscure substantial electron delocalization or accessible ligand-centred redox states.

A metal–organic framework is an extended coordination material built from metal-containing nodes joined by organic linkers. Many frameworks possess accessible pores, although porosity and stability depend on the particular structure.

Changing the linker or node can alter pore size, adsorption, and chemical functionality. This supports applications in molecular separation and gas uptake while showing why surface area alone is not a sufficient description of performance.

The set of ligands directly associated with a metal centre in the stated structural model. Distinguish primary donor bonds from secondary contacts and outer-sphere ions or lattice solvent.

Interconversion between electronic forms that differ in the formal distribution of oxidation among metal and ligands, without changing overall composition. A cobalt dioxolene example exchanges low-spin Co(III)–catecholate–semiquinonate and high-spin Co(II)–bis(semiquinonate) forms. This differs from changing only the spin state at fixed metal and ligand charges.

A repeating structure in which metal centres are connected by bridging ligands. Its coordination connectivity can extend along one, two or three independent directions.

A complex in which a metal is bound between two cyclopentadienyl-type rings, commonly with hapticity ${\eta }^5$ at each ring. In ionic electron counting, each unsubstituted cyclopentadienyl anion donates six electrons.

An orbital that contains an unpaired electron and dominates a center’s exchange interactions.

Transfer of an organic group from one element to a metal centre. A rhodium hydroxide and an arylboronic acid can exchange Rh-OH and B-Ar connectivity to form Rh-Ar and boric acid.

A coordination complex that retains donor sites capable of binding additional metal ions. Distinguishable internal and external coordination sites permit hierarchical construction of heterometallic assemblies.