Chapter 5 - Coordination Compounds

Coordination Compounds explains substances in which a central metal atom or ion is surrounded by ions or molecules called ligands. These compounds account for the colours of many transition-metal solutions, the action of haemoglobin and chlorophyll, several medicines, analytical tests and important metallurgical processes.

The chapter introduces a precise language for describing complexes, then uses bonding theories to explain their geometry, magnetic behaviour, colour and stability. It is a concept-rich chapter in which correct notation and systematic naming are especially important.

Werner's theory and basic terminology

Werner distinguished between primary and secondary valencies. In modern terms, primary valency corresponds broadly to oxidation state and is normally ionisable, while secondary valency corresponds to coordination number and is directed in space.

Important terms include central metal atom or ion, ligand, donor atom, coordination entity, coordination sphere, coordination number, oxidation number, homoleptic complex and heteroleptic complex.

Ligands may be neutral or charged and are classified by the number of donor atoms through which they bind. Monodentate ligands bind through one donor atom, bidentate ligands through two and polydentate ligands through several. Ambidentate ligands can coordinate through different possible donor atoms, while chelating ligands form ring structures with the metal.

Nomenclature of coordination compounds

Systematic naming follows an ordered set of rules. The cation is named before the anion. Within a coordination entity, ligand names appear before the metal name and are arranged alphabetically, ignoring numerical prefixes for alphabetisation. The oxidation state of the metal is written in Roman numerals.

Anionic metal complexes use a modified metal name ending in “-ate”; some metals use Latin-derived forms. Writing the formula from a name requires calculating charges carefully so that the complete compound is electrically neutral.

Isomerism

Coordination compounds show structural isomerism and stereoisomerism. Structural forms include ionisation, hydrate or solvate, linkage and coordination isomerism. Stereoisomers have the same bonding sequence but differ in spatial arrangement and include geometrical and optical isomers.

Square-planar and octahedral complexes commonly show geometrical isomerism. Optical isomers are non-superimposable mirror images. Drawing the geometry is far safer than deciding isomerism from the formula alone.

Bonding through valence bond theory

Valence bond theory describes ligand electron pairs occupying hybrid orbitals of the central metal. It predicts common geometries such as tetrahedral, square planar and octahedral and can estimate the number of unpaired electrons.

Inner-orbital and outer-orbital octahedral complexes differ in whether pairing allows inner d orbitals to participate. Although valence bond theory is useful for shape and simple magnetic predictions, it does not adequately explain colour or the full origin of strong- and weak-field behaviour.

Crystal field theory

Crystal field theory treats ligands as producing an electrostatic field that splits the metal d orbitals into groups of different energy. In an octahedral field, the d orbitals split into lower-energy t2g and higher-energy eg sets. In a tetrahedral field, the order is reversed and the splitting is smaller.

Whether electrons pair in the lower set or occupy higher orbitals depends on the competition between crystal-field splitting and pairing energy. This leads to high-spin and low-spin configurations. The d-electron arrangement then helps predict the number of unpaired electrons, magnetic behaviour and possible colour.

Colour and magnetic properties

Many coordination compounds absorb particular wavelengths of visible light for d-d transitions. The observed colour is complementary to the absorbed colour. The exact colour depends on the metal, oxidation state, ligand and geometry.

Magnetic behaviour follows from the presence or absence of unpaired electrons. Students should calculate the metal oxidation state, obtain the d-electron count, apply the correct splitting pattern and then count unpaired electrons.

Metal carbonyls and bonding

Metal carbonyls contain carbon monoxide ligands. Their bonding involves donation of an electron pair from CO to the metal together with back-bonding from filled metal d orbitals into antibonding orbitals on CO. This synergic interaction strengthens the metal-carbon bond.

Importance and applications

Coordination compounds are central to qualitative analysis, extraction of metals, electroplating, medicine and biological processes. Haemoglobin contains an iron complex, chlorophyll contains magnesium and vitamin B12 contains cobalt. Chelating agents can bind metal ions selectively, and coordination principles are used in cancer therapy and hardness estimation.

What can students study on Teachoo?

• Werner's theory and key definitions;

• ligand classification and coordination number;

• oxidation-state calculations;

• IUPAC names and formula writing;

• structural, geometrical and optical isomerism;

• valence bond theory and hybridisation;

• crystal-field splitting in octahedral and tetrahedral complexes;

• high-spin and low-spin arrangements;

• colour and magnetic moment;

• metal-carbonyl bonding; and

• applications of coordination compounds.

Common mistakes to avoid

• Do not confuse coordination number with oxidation state.

• Count donor atoms, not simply the number of ligand molecules, when finding coordination number.

• Arrange ligand names alphabetically without using multiplicative prefixes to decide the order.

• Determine metal oxidation state before finding its d-electron count.

• Do not assume every four-coordinate complex is tetrahedral; some are square planar.

• Draw structures before deciding geometrical or optical isomerism.

Best way to study Coordination Compounds

Learn the vocabulary first, because every later topic uses it. Practise ten names and ten formulas in both directions. Then study isomerism through drawings, followed by valence bond theory and crystal field theory. For every bonding question, use the same sequence: oxidation state, d count, coordination number, ligand field, orbital arrangement, geometry and magnetic behaviour.

Frequently asked questions

What is a ligand?

A ligand is an ion or molecule that donates at least one electron pair to a central metal atom or ion to form a coordinate bond.

What is coordination number?

It is the number of ligand donor atoms directly bonded to the central metal, not necessarily the number of ligand molecules.

What is the difference between a double salt and a complex compound?

A double salt largely dissociates into its constituent ions in solution. A complex retains a recognisable coordination entity containing the metal and its ligands.

What is chelation?

Chelation occurs when a multidentate ligand binds through multiple donor atoms and forms one or more rings with the central metal.

Why are some complexes high spin and others low spin?

The result depends on whether crystal-field splitting is smaller or larger than the energy required to pair electrons.

Why are many coordination compounds coloured?

They absorb selected wavelengths of visible light, often through transitions between split d orbitals, and the complementary light is observed.

How should magnetic behaviour be predicted?

Find the oxidation state and d-electron count, place the electrons in the appropriate crystal-field arrangement, and count the unpaired electrons.

Learn Coordination Compounds with Teachoo using systematic naming steps, clear isomer drawings and repeatable methods for bonding and magnetic questions.