Schooling · Class 12 · Chemistry · Coordination Compounds · Chapter on a page
CBSE Class 12 · Chemistry · NCERT Chemistry-I
Coordination Compounds: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 27 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
This note's current text has no applied fact-check on record.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- A coordination entity consists of a…
- Ligands are classified by the number…
- Werner's coordination theory…
- Coordination number is the total…
- Isomerism in coordination compounds…
- Valence bond theory uses…
- Crystal field theory treats ligands…
- Formulas and facts
- Oxidation state of metal = total…
- Coordination number = number of…
- IUPAC naming order
- Effective atomic number (EAN) =…
- Crystal field splitting energy Δₒ…
- Spectrochemical series (increasing Δ)
- High-spin complexes form when Δ is…
- Worked examples
- Finding oxidation state and…
- IUPAC naming
- Predicting geometry and magnetism…
- Watch out for
- Adding the number of ligand…
- Forgetting that neutral ligands…
- Naming ligands in the order they…
- Assuming all four-coordinate…
- Using the crystal field splitting…
What this chapter is about
Coordination compounds are substances in which a central metal atom or ion is surrounded by molecules or ions called ligands, which donate electron pairs to the metal. These compounds are essential in chemistry because they explain the colours of many transition-metal solutions, the functioning of haemoglobin and chlorophyll, and the extraction of metals like silver and gold. You meet this chapter now because it builds directly on your knowledge of d-block elements, chemical bonding and electron configurations from earlier study.
Key ideas
- A coordination entity consists of a central metal atom or ion bonded to a fixed number of ligands; the charge on the whole entity is the sum of the metal's oxidation state and the charges on the ligands.
- Ligands are classified by the number of donor atoms: monodentate (one, e.g. Cl⁻, NH₃), bidentate (two, e.g. ethylenediamine, en), polydentate (many, e.g. EDTA⁴⁻ with six donor atoms).
- Werner's coordination theory distinguishes primary valency (oxidation state of metal, satisfied by anions) from secondary valency (coordination number, satisfied by ligands in a definite geometry).
- Coordination number is the total number of donor atoms directly attached to the metal; common values are 4 (tetrahedral or square planar) and 6 (octahedral).
- Isomerism in coordination compounds includes structural types (ionisation, linkage, coordination) and stereoisomers (geometrical cis-trans, optical mirror-image forms).
- Valence bond theory uses hybridisation (sp³, dsp², sp³d², d²sp³) to predict geometry and magnetic properties based on whether electrons are paired or unpaired.
- Crystal field theory treats ligands as point charges that split the d-orbitals of the metal; in an octahedral field the five d-orbitals split into a lower t₂g set and a higher eg set, with splitting energy Δₒ.
- Colour arises when a complex absorbs visible light whose energy matches Δₒ (or Δₜ for tetrahedral), promoting a d-electron to a higher orbital; the transmitted light is the complementary colour.
Formulas and facts to remember
- Oxidation state of metal = total charge on complex − sum of charges on all ligands.
- Coordination number = number of ligand donor atoms bonded to the metal (not the number of ligand molecules).
- IUPAC naming order: cation first, then anion; within the coordination sphere, ligands are named alphabetically, with anionic ligands ending in -o (e.g. chlorido), neutral ligands by their molecule name (aqua for H₂O, ammine for NH₃).
- Effective atomic number (EAN) = atomic number of metal − oxidation state + 2 × coordination number; a value of 36, 54 or 86 (noble-gas configuration) often indicates extra stability.
- Crystal field splitting energy Δₒ for octahedral complexes is about 10 Dq; for tetrahedral, Δₜ ≈ (4/9)Δₒ.
- Spectrochemical series (increasing Δ): I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < NO₂⁻ < CN⁻ < CO.
- High-spin complexes form when Δ is small (weak-field ligands); low-spin complexes form when Δ is large (strong-field ligands), causing electrons to pair in the lower set.
- Chelate effect: polydentate ligands form more stable complexes than equivalent monodentate ligands because the entropy change on complex formation is more favourable.
Common mistakes
- Adding the number of ligand molecules instead of donor atoms when finding coordination number → count each donor atom, not each ligand molecule (en has 2 donor atoms).
- Forgetting that neutral ligands contribute zero charge when calculating oxidation state → always assign charge only to ionic ligands.
- Naming ligands in the order they appear in the formula rather than alphabetically → list ligand names in strict alphabetical order, ignoring prefixes like di-, tri-.
- Assuming all four-coordinate complexes are tetrahedral → with d⁸ metals and strong-field ligands, square planar geometry is common.
- Using the crystal field splitting value Δₒ for a tetrahedral complex → use Δₜ ≈ (4/9)Δₒ; tetrahedral splitting is smaller.
Quick revision
- Coordination number = total donor atoms bonded to the metal.
- Oxidation state = complex charge minus sum of ligand charges.
- Spectrochemical series ranks ligands by their ability to cause d-orbital splitting (I⁻ weakest, CO strongest).
- Strong-field ligands produce low-spin, often diamagnetic complexes; weak-field ligands produce high-spin, paramagnetic complexes.
- Colour of a complex depends on the energy gap Δ; absorbed light's complementary colour is observed.
- Chelating ligands form extra-stable complexes due to the chelate effect.