Is it correct to say: Treating coordination number as the oxidation state of the metal.?

CN counts donor atoms bonded to metal; OS is the metal's formal charge. In [Co(NH₃)₆]³⁺: CN = 6, OS = +3.

Is it correct to say: Naming ligands in random order instead of alphabetically.?

Ligands appear in ALPHABETICAL order in the name, regardless of charge. Multiplying prefixes (di, tri) are NOT counted for alphabetisation.

Is it correct to say: Using 'di' instead of 'bis' for complex ligand names.?

Use bis, tris, tetrakis when ligand name itself contains di/tri/tetra (e.g. bis(ethylenediamine), tris(ethylenediamine)).

Is it correct to say: Assuming every d⁴–d⁷ octahedral complex is low-spin.?

Depends on Δ_o vs pairing energy P. With weak-field ligands (H₂O, F⁻) they are high-spin; strong-field (CN⁻, CO) makes them low-spin.

Is it correct to say: Applying spin-only formula to complexes with strong orbital contribution (some lanthanides / 4d, 5d).?

For 3d complexes orbital contribution is 'quenched' — spin-only is a good approximation. For heavier elements, use effective moment including L-S coupling.

Is it correct to say: Confusing geometrical (cis/trans) with optical isomerism for square planar complexes.?

Cis/trans is geometrical (based on ligand positions). Square planar complexes generally lack a stereocentre and are NOT optically active. Optical isomerism is common in octahedral [M(bidentate)₃] and cis-[M(bidentate)₂X₂].

Coordination Compounds

Werner's theory, IUPAC nomenclature, VBT, crystal field theory (CFT), isomerism, magnetic behaviour — NCERT Class 12 Chemistry Ch 5

📐 10 formulas✏️ 3 examples🎯 6 practice⚖️ 6-8 marks🏫 CBSE📚 Class 12✓ 2025–26 syllabus

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Board Exam Tips

→IUPAC naming of complexes is a guaranteed 2-3 marker. Practise the order: ligands (alphabetical, with prefixes di/tri/bis/tris) → metal (oxidation state in Roman numerals).
→Spin-only magnetic moment μ = √n(n+2) BM — memorise: n=1 → 1.73, n=2 → 2.83, n=3 → 3.87, n=4 → 4.90, n=5 → 5.92 BM.
→Strong-field ligands (CN⁻, CO, NH₃) cause pairing → low-spin; weak-field (H₂O, F⁻, Cl⁻) → high-spin. Spectrochemical series is examinable.
→Distinguish coordination number (bonds to central atom) from oxidation state (charge on central atom) — a favourite MCQ trap.
→For d⁴–d⁷ octahedral complexes, high-spin vs low-spin depends on ligand field. For d¹–d³ and d⁸–d¹⁰ only one configuration is possible.

📐 Formulas(10)

Werner's Primary and Secondary Valency

Primary valency = oxidation state (ionisable); Secondary valency = coordination number (non-ionisable)

Oxidation State of Central Metal★ Board fav

Charge on complex = OS_{M} + \sum (ligand charges)

Symbol	Meaning
$OS_{M}$	Oxidation state of the central metal
$ligand charge$	Formal charge on each coordinated ligand

Coordination Number★ Board fav

CN = total donor atoms bonded to central metal

Spin-Only Magnetic Moment★ Board fav

μ = n (n + 2) BM

Symbol	Meaning
$n$	Number of unpaired electrons
$μ$	Magnetic moment (Bohr magnetons, BM)

Crystal Field Splitting (Octahedral)★ Board fav

Δ_{o} = E (e_{g}) - E (t_{2 g}), E (t_{2 g}) = - 0.4 Δ_{o}, E (e_{g}) = + 0.6 Δ_{o}

Crystal Field Stabilisation Energy (CFSE)

CFSE = [(- 0.4) n_{t_{2 g}} + (+ 0.6) n_{e_{g}}] Δ_{o} + P

Symbol	Meaning
$n_{t_{2 g}}$	Electrons in t₂g set
$n_{e_{g}}$	Electrons in eg set
$Δ_{o}$	Octahedral crystal field splitting (units of energy)
$P$	Pairing energy (added for each extra pair)

High-Spin vs Low-Spin Condition

Δ_{o} < P \Rightarrow high-spin; Δ_{o} > P \Rightarrow low-spin

Tetrahedral Splitting

Δ_{t} = \frac{4}{9} Δ_{o}

IUPAC Nomenclature Rule (order)★ Board fav

[Cation] [Anion] : ligands (alphabetical, with di/tri or bis/tris) + metal(OS in Roman)

Effective Atomic Number (EAN)

EAN = Z - OS_{M} + 2 \times CN

✏️ Solved Examples

1Solved Exampleeasy4 steps

For the complex [Co(NH₃)₆]³⁺: (i) find oxidation state of Co, (ii) coordination number, (iii) magnetic moment assuming NH₃ is a strong-field ligand.

NH₃ is neutral; overall complex charge +3

OS_{C o} + 6 (0) = + 3 \Rightarrow OS_{C o} = + 3

2Solved Exampleboard4 steps

Write the IUPAC name of [Cr(en)₂Cl₂]⁺ and give its magnetic moment. (Cr³⁺ = d³, en is neutral bidentate)

Determine oxidation state of Cr

OS_{C r} + 2 (0) + 2 (- 1) = + 1 \Rightarrow OS_{C r} = + 3

3Solved ExampleHOTS4 steps

Explain why [Fe(H₂O)₆]³⁺ is high-spin and coloured, while [Fe(CN)₆]³⁻ is low-spin and (nearly) colourless-to-lightly coloured. Compare their magnetic moments.

Both have Fe³⁺ = 3d⁵. H₂O is weak-field (small Δ_o), CN⁻ is strong-field (large Δ_o)

H_{2} O : Δ_{o} < P \Rightarrow high-spin; CN^{-} : Δ_{o} > P \Rightarrow low-spin

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Treating coordination number as the oxidation state of the metal.
✓CN counts donor atoms bonded to metal; OS is the metal's formal charge. In [Co(NH₃)₆]³⁺: CN = 6, OS = +3.
2
Naming ligands in random order instead of alphabetically.
✓Ligands appear in ALPHABETICAL order in the name, regardless of charge. Multiplying prefixes (di, tri) are NOT counted for alphabetisation.
3
Using 'di' instead of 'bis' for complex ligand names.
✓Use bis, tris, tetrakis when ligand name itself contains di/tri/tetra (e.g. bis(ethylenediamine), tris(ethylenediamine)).
4
Assuming every d⁴–d⁷ octahedral complex is low-spin.
✓Depends on Δ_o vs pairing energy P. With weak-field ligands (H₂O, F⁻) they are high-spin; strong-field (CN⁻, CO) makes them low-spin.
5
Applying spin-only formula to complexes with strong orbital contribution (some lanthanides / 4d, 5d).
✓For 3d complexes orbital contribution is 'quenched' — spin-only is a good approximation. For heavier elements, use effective moment including L-S coupling.
6
Confusing geometrical (cis/trans) with optical isomerism for square planar complexes.
✓Cis/trans is geometrical (based on ligand positions). Square planar complexes generally lack a stereocentre and are NOT optically active. Optical isomerism is common in octahedral [M(bidentate)₃] and cis-[M(bidentate)₂X₂].

🎯 Practice Yourself

🎯Practice Yourself6 questions

Q1
Write the IUPAC name of K₄[Fe(CN)₆].
Q2
For [Ni(CN)₄]²⁻ (Ni²⁺ = d⁸, strong-field), predict geometry, hybridisation and magnetic behaviour.
Q3
How many unpaired electrons are in high-spin [Mn(H₂O)₆]²⁺? Calculate μ.
Q4
Give the total number of geometrical isomers of [Co(NH₃)₄Cl₂]⁺.
Q5
Which of these ligands is NOT ambidentate: NO₂⁻, SCN⁻, CN⁻, en?
Q6
Explain why [Cu(NH₃)₄]²⁺ is coloured but [Zn(NH₃)₄]²⁺ is colourless.

📝 Notes

Coordination Compounds

Compounds in which a central metal atom / ion is surrounded by molecules or ions (ligands) that donate electron pairs to form coordinate bonds — a special kind of Lewis acid–base adduct.

Terminology

Complex ion: the charged metal-ligand unit inside square brackets, e.g. [Fe(CN)₆]³⁻.
Coordination sphere / entity: the whole species inside the brackets.
Counter ion: ion outside the brackets, balances charge, freely dissociates.
Ligand: electron pair donor. Classified by denticity: monodentate (Cl⁻, NH₃), bidentate (en, ox²⁻), polydentate (EDTA⁴⁻).
Chelate: cyclic complex formed by a polydentate ligand — extra stability due to ring formation.
Ambidentate: ligand that can attach through more than one donor atom (NO₂⁻/–ONO⁻, SCN⁻/–NCS⁻).

Werner's theory (1893)

Explained the two 'valencies':

Primary (ionisable): oxidation state, satisfied by anions outside the sphere.
Secondary (non-ionisable): coordination number, fixed for a metal, directional (gives geometry).

Bonding models

VBT (Valence Bond Theory): central metal uses hybrid orbitals (sp³ tetrahedral, dsp² square planar, sp³d² outer-orbital octahedral, d²sp³ inner-orbital octahedral) to accept ligand pairs. Explains geometry and magnetism but not colour.
CFT (Crystal Field Theory): ligands are point charges that split the d-orbitals in the metal's electric field. Explains colour (d–d transitions), magnetism (high- vs low-spin), and thermodynamic stability (CFSE) but treats bond as purely ionic.

Spectrochemical series (weak ↔ strong field)

I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < H₂O < NH₃ < en < NO₂⁻ < CN⁻ < CO

Ligands to the LEFT are weak-field (small Δ, high-spin); those to the RIGHT are strong-field (large Δ, low-spin).

Isomerism in complexes

Structural: ionisation, hydrate/solvate, linkage (ambidentate), coordination.
Stereoisomerism: geometrical (cis/trans, fac/mer), optical (mirror-image, chiral complexes).

Applications

Biological (haemoglobin — Fe(II) porphyrin; chlorophyll — Mg(II) porphyrin; vitamin B12 — Co(III)), analytical (EDTA titration for hardness of water), industrial (electroplating from [Ag(CN)₂]⁻), and metallurgical (extraction of Ag and Au as cyanide complexes, purification of Ni via Mond's process).