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Coordination Compounds

Naming complexes, counting isomers, and crystal field theory with spin-only magnetic moments. Many of its numeric answers are counts from a list.

Questions in the bank
215
q/paper in 2025–26
1.89
Calculation
35%
Notes pages
8

Strand: Structure and recall

When you’ll see it

A complex in square brackets: the ions outside the bracket, its name, isomers, hybridisation, d configuration, spin-only moment, CFSE, the wavelength it absorbs, or its use.

How this chapter is tested

Most questions run one routine: find the metal's oxidation state and d count, decide whether the ligand is strong or weak field, then read off what is asked: a hybridisation, the unpaired electrons, a spin-only moment, a CFSE or the wavelength absorbed. Learn it on cobalt, iron and nickel complexes and it covers the bulk of the chapter.

The numeric answers are counts with no slack: moles of AgCl from the chlorides outside the bracket, the stereoisomers of a formula type, the paramagnetic species in a list, a moment in the units the stem names. Count species, not electrons, and check whether the question wants electrons, pairs or unpaired electrons.

The recall half is exact: the NCERT spectrochemical series in order, which formula types allow cis–trans or fac–mer forms and which are chiral, and the named complexes (Ni(dmgH)₂, Wilkinson's catalyst, cisplatin, chlorophyll) with the metal each holds. A few keys follow a textbook rule even where measured data disagree; on the paper, answer by the rule.

The sub-skills

The distinct skills inside the chapter, in the order to learn them.

  • Werner's theory

    Only ions outside the bracket ionise: [Co(NH₃)₄Cl₂]Cl gives one AgCl; primary valency is the oxidation state, secondary valency the coordination number; a double salt gives all its simple ions.

  • Ligands and naming

    Ambidentate ligands (NO₂⁻, SCN⁻, CN⁻) bind through one of two atoms; chelating ones (en, C₂O₄²⁻, EDTA⁴⁻) through several at once; ligands go alphabetically, an anionic complex ends in -ate, the oxidation state in Roman numerals.

  • Isomerism

    Linkage, ionisation, coordination (two different metals) and solvate isomers; square planar MA₂B₂ and octahedral MA₄B₂ and MA₃B₃ have two geometrical forms; M(AA)₃ is chiral; stereoisomers = achiral forms + twice the chiral ones.

  • Valence bond theory

    A strong field pairs electrons: d²sp³, inner orbital, low spin; a weak field gives sp³d², high spin; [Ni(CN)₄]²⁻ is dsp² square planar, [NiCl₄]²⁻ and Ni(CO)₄ sp³ tetrahedral; octahedral Ni²⁺ is always sp³d².

  • Crystal field splitting and colour

    Octahedral eg +0.6Δₒ, t₂g −0.4Δₒ; tetrahedral inverted, with Δₜ = 4/9 Δₒ; I⁻ < Br⁻ < SCN⁻ < Cl⁻ < S²⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < EDTA⁴⁻ < NH₃ < en < CN⁻ < CO; a stronger field absorbs a shorter wavelength.

  • High and low spin and CFSE

    The choice exists only for d⁴ to d⁷; CFSE = (−0.4 × t₂g electrons + 0.6 × eg electrons)Δₒ, largest for low-spin d⁶ (−2.4Δₒ) and zero for high-spin d⁵; a tetrahedron fills e before t₂ and is always high spin.

  • Spin-only moment

    μ = √(n(n+2)) BM; low-spin d⁶ ([Fe(CN)₆]⁴⁻, [Co(NH₃)₆]³⁺), square planar d⁸ and d¹⁰ are diamagnetic; [Fe(CN)₆]³⁻ has one unpaired electron; Cu⁺ is diamagnetic, Cu²⁺ gives 1.73 BM.

  • Carbonyls, stability and uses

    CO σ-donates and π-accepts, so M–C strengthens and C–O weakens; Mn₂(CO)₁₀ has no bridging CO, Co₂(CO)₈ two; chelation raises stability; chlorophyll Mg, haemoglobin Fe, vitamin B₁₂ Co, cisplatin Pt, Wilkinson's Rh.

Traps to expect

Distractor shapes this chapter reuses. The Traps page covers the ones that cut across chapters.

  • Chloride inside the bracket counted

    In [Cr(H₂O)₄Cl₂]Cl only one of the three chlorides reaches the silver ion. The tripled count is always an option.

  • Low-spin d⁶ counted as paramagnetic

    [Fe(CN)₆]⁴⁻, [Co(NH₃)₆]³⁺ and [Co(C₂O₄)₃]³⁻ have every electron paired in t₂g. They are the commonest slip in a paramagnetic count.

  • The octahedral order in a tetrahedron

    In a tetrahedron e lies below t₂ and Δₜ = 4/9 Δₒ. Writing t₂ first, or giving a tetrahedral CFSE in Δₒ, gets the answer wrong.

  • Splitting, CFSE and wavelength mixed

    Δₒ is the gap the absorbed light matches; CFSE is that gap times a d-count factor. A stronger ligand gives a larger gap and a SHORTER absorbed wavelength.

  • Geometrical and stereo counts merged

    [Co(en)₂Cl₂]⁺ has two geometrical isomers but three stereoisomers, because the cis form is chiral. Tetrahedral complexes have no cis–trans forms at all.

Learn it before you drill it

This chapter has full teaching notes — foundations, worked examples, self-checks and a mastery check for each page. Read the notes once, then drill page by page below.

Coordination Compounds notes

Drill every Coordination Compounds question

215 questions from the bank, across 8 subtopics.

Drill the 215 questions

Drill one subtopic at a time

The 8 subtopics, in teaching order.

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