Coordination Chemistry Lecture Notes

Vocabulary terms and definitions covering coordination compounds, isomerism, Werner's theory, and bonding from the lecture notes.

Co-ordination Compound

A compound that results from the combination of two saturated compounds of different species and retains its identity in both the solid state and solution phase, possessing properties entirely different from those of its constituents.

Double Salt

A molecular compound that exhibits crystal lattices in the solid state but completely breaks down into its constituent compounds in solvents, displaying physical and chemical properties essentially identical to those of its constituents.

Ligand

An atom, molecule, or ion capable of forming a coordinate bond with the central metal atom or ion in a coordination compound.

Monodentate Ligand

A ligand having only one donor site that can form only one coordinate bond with the central metal atom or ion (e.g., $\text{NH}_3$, $\text{H}_2\text{O}$, $\text{CO}$, $\text{F}^-$, $\text{Cl}^-$).

Bidentate Ligand

A ligand possessing two donor sites that can form two coordinate bonds with the central metal atom or ion (e.g., ethane-1,2-diamine, oxalate).

Ambidentate Ligand

A ligand containing more than one potential donor site, but in which only one donor site is linked to the central metal atom or ion at a given time (e.g., $\text{SCN}^-$, $\text{NCS}^-$, $\text{NO}_2^-$, $\text{ONO}^-$).

Polydentate Ligand

A ligand possessing more than two donor sites, such as the hexadentate ligand EDTA.

Denticity

The number of ligating sites or coordinate bonds formed by a single ligand with the central metal atom or ion.

Co-ordination Number

The total number of monodentate ligands that surround the central metal atom or ion in a coordination compound.

Coordination Sphere

The central metal atom or ion together with its attached ligands, enclosed within square brackets.

Coordination Entity

A chemical unit consisting of a central metal atom or ion bonded to a fixed number of ligands.

Oxidation Number

A number representing the actual or apparent electrical charge an atom carries when in combination with other ions.

Charge on Complex Ion

The algebraic sum of the charges carried by the central metal atom or ion and its attached ligands.

Coordination Polyhedron

The spatial arrangement of ligands around the central metal atom or ion, commonly forming geometries such as octahedral, tetrahedral, or square planar.

Homoleptic Complex

A complex in which the central metal atom or ion is surrounded by only one type of ligand (e.g., $[\text{Co}(\text{NH}_3)_6]\text{Cl}_3$).

Heteroleptic Complex

A complex in which the central metal atom or ion is surrounded by more than one type of ligand (e.g., $[\text{Co}(\text{NH}_3)_4\text{Cl}_2]\text{Cl}$).

Primary Valency

The ionisable, non-directional valency in Werner's theory that a metal exhibits in simple salt formation, satisfied by negatively charged ions and corresponding to the modern oxidation state.

Secondary Valency

The non-ionisable, directional valency in Werner's theory that a metal directs toward ligands to impart definite geometry, corresponding to the modern coordination number.

Isomerism

The phenomenon in which two or more compounds possessing the same molecular formula have different structural or spatial arrangements of atoms.

Stereoisomers

Isomers sharing the same chemical formula and chemical bonding framework but possessing different spatial arrangements of their atoms.

Geometrical Isomerism

A form of stereoisomerism occurring in square planar and octahedral complexes where identical ligand groups occupy either adjacent positions (cis isomer) or opposite positions (trans isomer).

Facial (fac) Isomer

An isomer of an octahedral entity of type $\text{Ma}_3\text{b}_3$ in which three donor atoms of the same ligand occupy adjacent corners of an octahedral face.

Meridional (mer) Isomer

An isomer of an octahedral entity of type $\text{Ma}_3\text{b}_3$ in which three donor atoms of the same ligand are arranged around a meridian of the octahedron.

Optical Isomers

Non-superimposable mirror image isomers, also referred to as enantiomers.

Dextrorotatory ('d') Isomer

An optical isomer that rotates the plane of plane-polarized light toward the right.

Laevorotatory ('l') Isomer

An optical isomer that rotates the plane of plane-polarized light toward the left.

Ionisation Isomerism

A structural isomerism in which isomers produce different ions in solution because the position of counter ions and ligands differs inside and outside the coordination sphere.

Hydrate Isomerism

A structural isomerism in which compounds with the same composition differ in the number of water molecules bound as coordinating ligands versus water molecules present outside the coordination sphere as crystal lattice hydration.

Linkage Isomerism

A structural isomerism occurring in complexes containing ambidentate ligands that can coordinate to the central metal atom through different donor atoms.

Coordination Isomerism

A structural isomerism that arises in compounds containing both cationic and anionic complex ions through the interchange or redistribution of ligands between the two metal centers.

Valence Bond Theory

A bonding theory proposing that the central metal atom or ion uses $(n-1)d$, $ns$, $np$ or $ns$, $np$, $nd$ orbitals under ligand influence to form hybrid orbitals of definite geometry that overlap with ligand orbitals donating electron pairs.