Coordination Compounds – Key Vocabulary

Vocabulary flashcards covering essential terms and definitions related to coordination compounds, ligands, geometries, valencies, types of complexes, Werner’s theory, and major forms of isomerism.

Coordination Compound

A compound containing a central metal ion surrounded by ligands joined through coordinate (dative) bonds.

Double Salt

A crystalline salt that contains two different simple salts and dissociates completely into all constituent ions in water.

Complex (Coordination) Salt

A salt that retains its identity in solution because the metal–ligand unit remains intact; shows primary and secondary valencies.

Coordinate (Dative) Bond

A covalent bond in which both electrons are donated by the same atom (the ligand) to the metal centre.

Central Metal Atom/Ion

The metal species in a coordination entity that accepts electron pairs from ligands and acts as a Lewis acid.

Ligand

An ion or molecule that donates one or more lone pairs of electrons to a central metal atom/ion to form a coordinate bond; behaves as a Lewis base.

Coordination Entity

The central metal atom/ion together with a fixed number of ligands arranged in a definite geometry, written inside square brackets.

Primary Valency

Ionisable valence (oxidation state) of the central metal; satisfied only by negative ions.

Secondary Valency

Non-ionisable valence of the metal corresponding to coordination number; satisfied by ligands and directional in nature.

Monodentate Ligand

A ligand that donates only one lone pair to the metal, e.g., NH₃, H₂O, Cl⁻, CN⁻.

Bidentate Ligand

A ligand that donates two lone pairs through two donor atoms, e.g., ethane-1,2-diamine (en), oxalate (C₂O₄²⁻).

Polydentate Ligand

A ligand capable of donating three or more lone pairs to a single metal ion, e.g., EDTA⁴⁻ (hexadentate).

Chelating Ligand

A di- or polydentate ligand that forms two or more coordinate bonds to the same metal, creating a ring (chelate).

Chelate Complex

A coordination compound in which a chelating ligand forms one or more rings with the central metal ion.

Ambidentate Ligand

A ligand possessing two or more different donor atoms but able to coordinate through only one at a time, e.g., NO₂⁻ (nitro / nitrito), SCN⁻ (thiocyanato / isothiocyanato).

Denticity

The number of donor sites (ligating atoms) present in a single ligand molecule.

Coordination Number

The total number of coordinate bonds (or equivalent monodentate ligands) attached to the central metal atom/ion.

Coordination Sphere

The part of the complex inside square brackets containing the central metal and its bound ligands.

Counter Ion

The ion outside the coordination sphere that balances the charge of the complex ion, e.g., K⁺ in K₄[Fe(CN)₆].

Coordination Polyhedron

The spatial arrangement (geometry) of ligands around the central metal, such as octahedral or tetrahedral.

Homoleptic Complex

A complex containing only one type of ligand around the metal centre, e.g., [Ni(CO)₄].

Heteroleptic Complex

A complex containing two or more different kinds of ligands, e.g., [Pt(NH₃)₂Cl₂].

Complex Ion

A charged coordination entity, such as [Co(NH₃)₆]³⁺ or [Fe(CN)₆]⁴⁻.

Cationic Complex

A complex ion carrying a positive charge, e.g., [Cu(NH₃)₄]²⁺.

Anionic Complex

A complex ion carrying a negative charge, e.g., [Fe(CN)₆]³⁻.

Neutral Complex

A coordination entity that is overall electrically neutral, e.g., [Ni(CO)₄].

Werner’s Theory

Early theory of coordination compounds proposing primary (ionisable) and secondary (non-ionisable) valences to explain structures and reactivity.

Ionisation Isomerism

Structural isomerism where compounds have the same formula but yield different ions in solution, e.g., [Co(NH₃)₅SO₄]Br vs. [Co(NH₃)₅Br]SO₄.

Linkage Isomerism

Isomerism arising when an ambidentate ligand coordinates through different donor atoms (e.g., nitro vs. nitrito).

Solvate (Hydrate) Isomerism

Isomerism in which water (or another solvent) molecule may be inside or outside the coordination sphere.

Coordination Isomerism

Occurs in compounds where both cation and anion are coordination entities; interchange of ligands between metal centres gives different isomers.