# Coordination Compounds

> Werner's theory, nomenclature, isomerism, VBT, crystal field theory and applications — NCERT Class 12 NEET-UG Chemistry notes.

- Canonical URL: https://prepelephant.com/topics/neet-ug/chemistry/coordination-compounds-ncert
- Exam / course: NEET-UG · Subject: Chemistry
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- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Coordination Compounds", PrepElephant, https://prepelephant.com/topics/neet-ug/chemistry/coordination-compounds-ncert

## Direct answer

Werner proved coordination chemistry with CoCl3·xNH3 compounds: 6NH3 releases 3 chloride ions in solution, 5NH3 releases 2, 4NH3 releases 1 — direct counts from silver chloride precipitation that established a fixed coordination sphere around cobalt. The examinable core is isomerism (ionisation, linkage, hydrate, geometrical, optical), Werner's legacy formalised in IUPAC naming, and two bonding models: valence bond theory's inner (d2sp3) versus outer (sp3d2) orbital complexes, and crystal field theory's Δo splitting, where strong-field ligands like CN− pair electrons into low-spin complexes and weak-field ligands like F− leave them unpaired. cis-Platin fights cancer; EDTA titrates water hardness; chlorophyll and haemoglobin are coordination compounds of magnesium and iron.

## What you must remember

- Ligand counts: bidentate (en, oxalate), hexadentate EDTA; chelate complexes are extra stable — the chelate effect.
- Naming: ligands alphabetically before the metal, anionic complexes end in -ate ([Fe(CN)6]^4− is hexacyanidoferrate(II)); oxidation state in Roman numerals.
- Ionisation isomers: [Co(NH3)5Br]SO4 versus [Co(NH3)5SO4]Br differ in which ligand precipitates with BaCl2 or AgNO3.
- Linkage isomerism needs ambidentate ligands — NO2− (N-bonded nitro) versus ONO− (O-bonded nitrito).
- Square planar MA2B2 complexes (cis/trans) and octahedral MA2B2C2 show geometrical isomerism; [Co(en)3]^3+ is chiral with optical isomers.
- Spectrochemical series (weak to strong): I− < Br− < Cl− < F− < OH− < H2O < NH3 < en < CN− < CO.
- Low spin forms when Δo exceeds the pairing energy P; high spin when it does not.
- Colour: [Ti(H2O)6]^3+ is violet from a d–d transition absorbing green; the observed colour is complementary to the absorbed light.

## High spin or low spin? A decision walked through

Compare [Co(NH3)6]^3+ and [CoF6]^3−. Cobalt(III) is d6: remove three electrons from [Ar]3d7 4s2 and you land on 3d6. In the fluoride complex, F− is weak field — Δo is smaller than the pairing energy — so the six electrons spread out t2g4 eg2 with four unpaired electrons: high spin, paramagnetic, and VBT calls it outer-orbital sp3d2. In the hexaammine complex, NH3 is strong enough that Δo beats pairing: the six electrons pair into t2g6, zero unpaired, diamagnetic, inner-orbital d2sp3. The decision is never about the metal alone — the same Co3+ ion flips its magnetism with the ligand, which is the entire point of the spectrochemical series. Answer the paired-unpaired question first, and magnetism, hybridisation and even colour intensity follow from it. When an exam question gives magnetic data instead (for instance, diamagnetic), reverse the logic to read off the ligand's field strength.

## The nickel pair and other traps

The nickel pair is the examiner's favourite contrast: [Ni(CN)4]^2− is square planar and diamagnetic (CN− strong, dsp2), while [NiCl4]^2− is tetrahedral and paramagnetic (Cl− weak, sp3) — same metal, same oxidation state, opposite answers. Second, naming order: ligands are cited alphabetically ignoring their di/tri prefixes (tetraammine before chlorido in [Co(NH3)4Cl2]+), and forgetting the -ate ending for an anionic metal is the commonest lost mark. Third, linkage isomerism requires an ambidentate ligand — NO2−, SCN−, CN−? The first two qualify; CN− also can, but NCERT's set examples are NO2− and SCN−. Fourth, hydrate (solvate) isomerism in CrCl3·6H2O differs from ionisation isomerism by swapping water across the coordination sphere boundary. Finally, [Co(en)3]^3+ shows optical isomerism without any carbon stereocentre — chirality belongs to geometry, not to carbon alone.

## Frequently asked questions

### How many chloride ions precipitate from CoCl3·5NH3 with silver nitrate?

Two, since the compound is [Co(NH3)5Cl]Cl2 — one chloride sits inside the coordination sphere and does not ionise.

### Which isomerism does the NO2− ligand show?

Linkage isomerism, binding through nitrogen (nitro) or oxygen (nitrito), as in the red and yellow isomers of [Co(NH3)5(NO2)]^2+.

### Why is [Ni(CN)4]^2− square planar but [NiCl4]^2− tetrahedral?

CN− is a strong-field ligand that pairs Ni(II) d8 electrons allowing dsp2 square planar geometry; weak-field Cl− leaves unpaired electrons and sp3 tetrahedral geometry.

### When do electrons pair in an octahedral crystal field?

When the splitting energy Δo exceeds the pairing energy P — strong-field ligands give low-spin paired complexes, weak-field ligands give high-spin complexes.

### Which coordination compound is used in cancer treatment?

cis-Platin, cis-[Pt(NH3)2Cl2], whose cis geometry lets both chloride ligands leave and cross-link tumour cell DNA.