Unit 5: Coordination Compounds - Vocabulary Flashcards

Key vocabulary terms, definitions, and concepts from Unit 5 on Coordination Compounds including Werner's theory, ligand classifications, isomerism, and bonding theories.

Coordination Compounds

Complex compounds in which central metal atoms or ions are bound to a number of anions or neutral molecules by sharing electrons.

Primary Valence

In Werner's theory, the ionisable linkages of a metal ion in a coordination compound that are satisfied by negative ions.

Secondary Valence

In Werner's theory, the non-ionisable linkages satisfied by neutral molecules or negative ions, which correspond to the fixed coordination number of the metal.

Coordination Entity

A species constituting a central metal atom or ion bonded to a fixed number of ions or molecules, such as $[CoCl_3(NH_3)_3]$.

Central Atom or Ion

The central atom/ion in a coordination entity to which a fixed number of ligands are bound in a definite geometry, acting as a Lewis acid.

Ligand

An ion or molecule bound directly to the central metal atom or ion in a coordination entity.

Unidentate Ligand

A ligand that binds to a central metal ion through a single donor atom, such as $Cl^-$ or $NH_3$.

Didentate Ligand

A ligand that can bind to a central metal ion through two donor atoms simultaneously, such as ethane-1,2-diamine ($en$) or oxalate ($C_2O_4^{2-}$).

Polydentate Ligand

A ligand possessing several donor atoms that can bind to a single central metal ion, such as hexadentate $EDTA^{4-}$.

Chelate Ligand

A di- or polydentate ligand that uses two or more donor atoms simultaneously to bind a single metal ion, forming a stable cyclic structure.

Ambidentate Ligand

A ligand possessing two different donor atoms that can coordinate to a central metal atom through either atom, such as $NO_2^-$ or $SCN^-$.

Coordination Number

The total number of ligand donor atoms to which a central metal ion is directly bonded via $σ$ bonds.

Coordination Sphere

The central atom/ion and its attached ligands enclosed within square brackets, forming a non-ionisable unit in solution.

Counter Ions

The ionisable species located outside the square brackets of a coordination sphere, such as $K^+$ in $K_4[Fe(CN)_6]$.

Coordination Polyhedron

The spatial arrangement of ligand donor atoms directly attached to the central metal atom or ion.

Homoleptic Complex

A complex in which a central metal atom or ion is bound to only one kind of donor group, such as $[Co(NH_3)_6]^{3+}$.

Heteroleptic Complex

A complex in which a central metal atom or ion is bound to more than one kind of donor group, such as $[Co(NH_3)_4Cl_2]^+$.

Geometrical Isomerism

A type of stereoisomerism in heteroleptic complexes arising from different possible spatial arrangements of ligands (e.g., $cis$ and $trans$ forms).

Facial ($fac$) Isomer

An octahedral geometrical isomer in which three identical donor atoms occupy adjacent positions at the corners of an octahedral face.

Meridional ($mer$) Isomer

An octahedral geometrical isomer in which three identical donor atoms occupy positions around the meridian of the octahedron.

Optical Isomers

Non-superimposable mirror image stereoisomers (enantiomers) that rotate the plane of polarized light in opposite directions.

Linkage Isomerism

Structural isomerism occurring in coordination compounds with ambidentate ligands that can bind through different donor atoms (e.g., $M-NO_2$ vs $M-ONO$).

Coordination Isomerism

Isomerism arising from the interchange of ligands between cationic and anionic entities of different metal ions in a complex salt.

Ionisation Isomerism

Isomerism arising when a counter ion in a complex salt displaces a ligand inside the coordination sphere to become a ligand itself.

Solvate Isomerism

Isomerism (also known as hydrate isomerism when water is involved) where solvent molecules differ in whether they are directly bonded to the metal ion or present as free solvent in the crystal lattice.

Crystal Field Splitting

The removal of degeneracy of $d$-orbitals in a central metal ion due to asymmetrical electrostatic repulsion from surrounding ligands, splitting them into sets of different energy.

Spectrochemical Series

An experimentally determined series ranking ligands in order of increasing field strength and crystal field splitting energy ($\Delta_o$).

Synergic Bonding

A unique self-reinforcing bond in metal carbonyls comprising ligand-to-metal $\sigma$-donation and metal-to-ligand $\pi$-back-donation.