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Coordination chemistry is where many students lose easy marks: a name with "amine" instead of "ammine", ligands in the wrong order, a missing "-ate". The rules are fixed and short. This post sets them out with worked names, then covers the isomerism and crystal field questions JEE and NEET ask most.
What does the syllabus cover?
The JEE (Main) 2026 syllabus lists Werner's theory, ligands, coordination number, denticity, chelation, "IUPAC nomenclature of mononuclear co-ordination compounds, isomerism" and "basic ideas of Crystal field theory, colour and magnetic properties". The NEET (UG)-2026 syllabus has the same list. The ideas go back to Alfred Werner, who won the 1913 Nobel Prize in Chemistry "in recognition of his work on the linkage of atoms in molecules".
How do you name a complex step by step?
The NCERT Class 12 chapter on Coordination Compounds gives the rules, based on IUPAC recommendations:
Name the cation first, whether or not it is the complex.
Inside the complex, list ligands in alphabetical order before the metal. Multiplying prefixes (di, tri, tetra) do not count for the order.
Anionic ligands end in -ido: chlorido, cyanido, hydroxido. NCERT notes that newer IUPAC recommendations changed the ending, so chloro became chlorido. Neutral ligands keep their names, except aqua (H₂O), ammine (NH₃), carbonyl (CO) and nitrosyl (NO).
For ligands whose names already contain a number, such as ethane-1,2-diamine, use bis, tris, tetrakis and brackets.
Give the metal's oxidation state in Roman numerals. If the complex is an anion, the metal name ends in -ate, with Latin roots for some (ferrate for iron, argentate for silver).
IUPAC has also published a short brief guide to inorganic nomenclature that summarises its 2005 recommendations.
Worked names.
[Co(NH₃)₅Cl]Cl₂. Two chloride ions outside, so the complex is +2. Five neutral NH₃ and one Cl⁻ inside, so Co is +3. Ammine (a) comes before chlorido (c). Name: pentaamminechloridocobalt(III) chloride.
K₃[Fe(CN)₆]. Three K⁺, so the complex is −3. Six CN⁻ give −6, so Fe is +3. The complex is an anion. Name: potassium hexacyanidoferrate(III).
[Pt(NH₃)₂Cl₂]. Neutral complex, Pt is +2. Order by "ammine" and "chlorido", not "di". Name: diamminedichloridoplatinum(II).
[CoCl₂(en)₂]⁺. NCERT's example: dichloridobis(ethane-1,2-diamine)cobalt(III).
Which kinds of isomerism do you need?
NCERT divides isomers into two families.
Structural isomers have different bonds:
Linkage: an ambidentate ligand binds through different atoms, such as NO₂⁻ through N (nitrito-N) or O (nitrito-O).
Ionisation: a ligand and a counter-ion swap, as in [Co(NH₃)₅(SO₄)]Br and [Co(NH₃)₅Br]SO₄. One gives a precipitate with BaCl₂, the other with AgNO₃.
Coordination: ligands swap between a complex cation and a complex anion.
Solvate (hydrate): water inside versus outside the coordination sphere.
Stereoisomers have the same bonds in different arrangements:
Geometrical: square planar [Pt(NH₃)₂Cl₂] has cis and trans forms. Octahedral MA₄B₂ has cis and trans; MA₃B₃ has fac and mer. Tetrahedral MA₂B₂ has no geometrical isomers, because every corner is next to every other.
Optical: complexes such as [Co(en)₃]³⁺ are non-superimposable on their mirror images.
How does crystal field theory predict magnetism?
In an octahedral field, the NCERT chapter explains, the five d orbitals split into a lower set of three (t₂g) and a higher set of two (eg), separated by Δₒ. From d⁴ onwards, each new electron has a choice: pair up in t₂g, paying the pairing energy P, or go up to eg, paying Δₒ. If Δₒ < P, you get a high-spin complex; if Δₒ > P, low-spin.
Which way it goes depends on the ligand. NCERT gives the spectrochemical series, from weak to strong field: I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < edta⁴⁻ < NH₃ < en < CN⁻ < CO.
Worked example. Co³⁺ is d⁶. With F⁻, a weak-field ligand, [CoF₆]³⁻ is high spin, t₂g⁴eg², with 4 unpaired electrons, so it is paramagnetic. The spin-only moment is √(4 × 6) = √24 ≈ 4.90 BM. With NH₃, [Co(NH₃)₆]³⁺ is low spin, t₂g⁶, with no unpaired electrons, so it is diamagnetic. NCERT describes both.
Which mistakes cost marks?
"Amine" for NH₃. The ligand is ammine, with two m's.
Alphabetising by prefix, putting dichlorido before ammine.
Forgetting -ate for anionic complexes.
Old ligand names where your paper uses the current ones; know both.
Counting geometrical isomers for tetrahedral MA₂B₂.
Our guide to learning inorganic chemistry with retrieval shows how to turn the ligand names and the spectrochemical series into self-test cards.
A practice routine
Day 1: name ten complexes from formulas, writing the charge line first.
Day 2: write ten formulas from names.
Day 3: for six complexes, list every possible isomer and sketch the geometrical ones.
Day 4: for six complexes, fill in the d electrons, decide high or low spin and give the spin-only moment.
Day 5: a mixed set of ten under time.
Octahedral isomers are hard to see on paper. Our post on VSEPR and molecular shapes builds the same 3D habits, and the Fermi Concept Library has every concept in chemistry, each with its own lesson and its place in the map, for when crystal field splitting itself is unclear.
Today, name [Cr(NH₃)₄Cl₂]Cl and decide whether it has geometrical isomers.
