Ch 5Coordination Chemistry — Class 12 Chemistry, concept-first.

The previous unit noted that transition metals have a tendency to form complexes, or coordination compounds -- a name built from the Latin complexus ('hold') and 'coordinate' ('to arrange').

When two or more stable compounds in solution are mixed together and allowed to evaporate/crystallise, the result is sometimes a double salt and sometimes a coordination compound, and the two look dec…

Swiss chemist Alfred Werner was the first to propose a theory of coordination compounds explaining their observed behaviour, put forward in 1893 -- remarkably, before the electron itself had even been…

The one major gap Werner's otherwise powerful theory left open: even though it explains primary/secondary valence, ionisable versus non-ionisable groups, and the geometry implied by a given secondary…

The working vocabulary needed to read and write coordination formulas correctly -- coordination entity, central atom, ligand and donor atom, coordination sphere and counter ion, coordination polyhedro…

A coordination entity is an ion or a neutral molecule made up of a central atom (usually a metal) together with the array of other atoms or groups of atoms, called ligands, that are directly attached…

The central atom or ion is the one occupying the central position in a coordination entity, binding the surrounding ligands to itself through coordinate covalent bonds -- for example Fe²⁺ in K₄[Fe(CN)…

Ligands are the atoms or groups of atoms bound to the central atom or ion, and the specific atom within a ligand that is directly bonded to the central metal is called the donor atom -- it is the dono…

The complex ion of a coordination compound -- the central metal atom/ion together with all the ligands directly attached to it -- is collectively called the coordination sphere, and it is conventional…

The coordination polyhedron (or coordination polygon) is the three-dimensional spatial arrangement traced out by the ligand donor atoms/ions that are directly attached to the central atom.

The coordination number of a metal in a complex is the number of ligand donor atoms bonded to the central metal ion -- equivalently, the number of sigma bonds formed between the ligands and the centra…

The oxidation state (or oxidation number) of the central atom in a coordination entity is the charge it would carry if all the ligands were removed together with the electron pairs that were shared wi…

Coordination compounds are classified in two independent ways: by the net charge carried by the complex ion, and by how many distinct kinds of ligand surround the central atom.

Before IUPAC standardised things, coordination compounds were simply named after their discoverers -- K[PtCl₃(C₂H₄)] was 'Zeise's salt' and [Pt(NH₃)₄][PtCl₄] was 'Magnus's green salt'.

Simple ions occurring in a coordination compound's formula are named exactly as in any other ionic compound: the simple cation keeps the element's ordinary name (sodium for Na⁺, potassium for K⁺, copp…

To name a complex ion, every ligand is named first, followed by the central metal atom/ion; when more than one kind of ligand is present, the ligand names are listed in strict alphabetical order (igno…

In cationic or neutral complexes the central metal keeps its ordinary element name (e.g. chromium, iron, cobalt); in anionic complexes a suffix '-ate' is added to the metal name instead (chromate, fer…

Three fully worked examples showing every step of building an IUPAC name -- identify the cation and anion, name each ligand with its correct multiplying prefix and κ-term where needed, alphabetise, na…

Just as organic compounds show isomerism, coordination compounds with the very same molecular formula can differ in how their ligands are arranged around the central metal atom, giving rise to compoun…

Coordination compounds sharing the same formula but differing in how their constituent atoms are actually connected to one another are called structural isomers (or constitutional isomers).

Linkage isomers arise when an ambidentate ligand -- one with two different possible donor atoms -- is bonded to the central metal through one donor atom in one isomer and through the other donor atom…

Coordination isomers arise in coordination compounds where BOTH the cation and the anion are themselves complex ions; interchanging one or more ligands between the cationic and anionic coordination en…

Ionisation isomers arise when an ionisable counter ion (a simple ion sitting outside the coordination sphere) is itself capable of acting as a ligand; exchanging that counter ion with one or more liga…

Solvate isomers arise when a free solvent molecule sitting in the crystal lattice (water, ammonia, alcohol, etc.) is exchanged with a ligand already inside the coordination entity, giving different is…

Stereoisomers share the same chemical formula and the same connectivity between the central metal atom and its ligands, differing only in the spatial arrangement of the ligands in three-dimensional sp…

Coordination compounds that possess chirality exhibit optical isomerism just like organic compounds do.

Werner treated bonding in coordination compounds as a Lewis acid-Lewis base interaction, which explains many observed properties but not colour or magnetism.

VBT explains the metal-ligand bond as arising from the overlap of a filled ligand orbital (holding a lone pair) with a vacant hybrid orbital on the central metal atom.

Crystal Field Theory (CFT) was developed to cover the colour and magnetic behaviour that VBT cannot explain.

In an octahedral field, six ligands approach the central metal ion along the +x, -x, +y, -y, +z and -z axes.

In a tetrahedral field, the four ligands can be visualised approaching along four alternate corners of a cube that has the metal ion at its centre -- i.e. along the cube's leading body diagonals.

The size of the crystal field splitting energy depends not only on whether the field is octahedral or tetrahedral, but also on the identity of the ligand, the identity of the central metal, and its ch…

Most transition-metal complexes are coloured because they absorb light of a particular wavelength from the visible region and transmit the rest; the colour we perceive is the complementary colour of t…

Metal carbonyls are transition-metal complexes of carbon monoxide containing a direct metal-carbon bond, in which the CO molecule acts as a neutral ligand; the first homoleptic carbonyl, nickel tetrac…

The stability of a coordination complex can be described in two different ways. Thermodynamic stability refers to the free energy change (ΔG) of the complex-formation reaction -- how favourable it is…

A free metal ion in aqueous medium is already surrounded by (coordinated to) water molecules, written [MS₆]; adding a ligand that binds more strongly than water displaces the coordinated water molecul…

Coordination complexes are of great importance and occur widely in plants, animals and minerals, alongside a long list of industrial, analytical and medicinal applications.

Cisplatin, cis-[Pt(NH₃)₂Cl₂], is a square planar platinum(II) coordination complex in which the two identical ligands of each pair (the two NH₃ and the two Cl) occupy positions adjacent to each other…

The chapter-end exercise: 20 multiple-choice questions covering Werner's theory and primary/secondary valence, IUPAC nomenclature, all four types of structural isomerism plus geometrical and optical i…

Sample papers and previous-year board questions for this subject.