Foundations, Nomenclature & Structural Isomerism — formula sheet

Primary valence (Hauptvalenz): the ionisable valence. It is satisfied only by anions, it equals the metal’s oxidation state, and it is non-directional — it says nothing about where anything sits in space. In modern language: the charge that must be balanced by counter-ions.

Secondary valence (Nebenvalenz): the non-ionisable valence. It is satisfied by neutral molecules or anions, it is fixed in number for a given metal ion, and it is directional — the groups satisfying it occupy definite positions in space. In modern language: the coordination number, and the geometry that goes with it.

Central atom (or central ion): the atom that accepts electron pairs — the Lewis acid of the assembly. Usually a d- or f-block metal, but not necessarily a metal at all (BF₄⁻ and SiF₆²⁻ are coordination entities by the same logic).

Ligand: an ion or molecule bound to the central atom through at least one donor atom — the Lewis base. From ligare, to bind.

Donor atom: the specific atom of the ligand that carries the lone pair actually used in bonding. In NH₃ it is nitrogen; in SCN⁻ it may be either sulfur or nitrogen, which is why such ligands get their own name (A.4).

Coordination number (CN): the number of donor atoms directly bonded to the central atom. Count donor atoms, never ligand molecules.

Coordination polyhedron: the geometrical figure traced by the donor atoms — octahedron, tetrahedron, square plane, and so on.

Coordination entity: the whole species inside the brackets, whether it is cationic ([Co(NH₃)₆]³⁺), anionic ([Fe(CN)₆]³⁻) or neutral ([Ni(CO)₄]).

Denticity: the number of donor atoms of a single ligand that bind to the same metal centre. From dens, tooth — a bidentate ligand bites with two teeth.

Hapticity (η^n): the number of contiguous atoms of a ligand’s π system that interact with the metal. Written as a superscript on eta: η²-ethene, η⁵-cyclopentadienyl, η⁶-benzene.

Ambidentate ligand: a ligand with two or more chemically different donor atoms, of which it uses only one at a time. The metal chooses; different choices give linkage isomers (Part C).

Bridging ligand (μ_n): a ligand bound simultaneously to n metal centres. Written μ₂, μ₃, and so on; the subscript is often omitted when it is 2.

Chelate: a complex in which a single ligand binds one metal through two or more donor atoms, forming a ring that includes the metal. From chele, the claw of a crab.

Macrocyclic effect: the extra thermodynamic stability of a complex of a cyclic polydentate ligand relative to its open-chain analogue with the same donor set.

Effective atomic number (EAN): the total electron count on the metal after coordination — its own electrons, adjusted for charge, plus two for every electron pair donated.

18-electron rule: stable organometallic complexes tend to have 18 valence electrons, filling all nine metal valence orbitals. Also called the noble-gas or effective atomic number rule.

Bite angle: the L–M–L angle subtended at the metal by the two donor atoms of a bidentate ligand. It is set largely by the ligand backbone, and a mismatch between the bite angle and the ideal angle for the geometry strains the complex.

Structural (constitutional) isomers: compounds with the same molecular formula in which the atoms are connected differently — different bonds, not merely a different arrangement in space.

Coordination-position isomerism: in a bridged dinuclear complex, the same ligands are distributed differently between the two metal centres while the bridge stays intact.