Is it correct to say: Naming cationic complexes with -ate suffix?

-ate suffix is only for anionic complexes: e.g. ferrate, cuprate. Cationic keeps the metal name.

Is it correct to say: Assuming coordination number = oxidation state?

CN is number of ligand atoms bonded; oxidation state is the charge on the metal. Different concepts.

Is it correct to say: Writing NH₃ as a strong-field ligand always?

NH₃ is moderate to strong. CN⁻ and CO are strongest. Refer spectrochemical series.

Is it correct to say: Using Δ_t = Δ_o (not 4/9 Δ_o)?

Tetrahedral field splitting is roughly 4/9 of octahedral; and t₂ (upper) > e (lower) in tetrahedral.

Is it correct to say: Applying spin-only formula μ = √(n(n+2)) with n = total electrons?

n is number of UNPAIRED electrons only. Paired ones contribute zero to spin.

Is it correct to say: Confusing ionization isomerism with linkage isomerism?

Ionization: swap ion inside/outside coord sphere. Linkage: same ligand bonds through different atoms (e.g. NO₂⁻ vs ONO⁻).

Coordination Compounds

Werner's theory, IUPAC nomenclature, isomerism, VBT, CFT — Maharashtra HSC Chemistry Ch 9

📐 11 formulas✏️ 3 examples🎯 5 practice⚖️ 5 marks🏫 Maharashtra State Board📚 Class 12✓ 2025–26 syllabus

💡

Board Exam Tips

→IUPAC naming of coordination compounds is a mandatory HSC skill — practise both cationic and anionic complexes with correct suffixes.
→Werner's postulates + example [Co(NH₃)₆]Cl₃ derivation of primary and secondary valencies is a 3-marker in Maharashtra Board.
→Hybridization and geometry of common complexes ([Ni(CN)₄]²⁻ dsp², [NiCl₄]²⁻ sp³, [Co(NH₃)₆]³⁺ d²sp³) is asked every year.
→Crystal Field Splitting (Δ_o for octahedral, Δ_t for tetrahedral) and magnetic behaviour prediction (paramagnetic/diamagnetic) is a Maharashtra HSC HOTS question.
→Distinguish clearly between double salt and coordination compound in a 2-marker.

📐 Formulas(11)

Werner's Primary and Secondary Valency★ Board fav

Primary = ionisable (satisfies charge), Secondary = coordination number

Coordination Number

C N = number of ligand donor atoms bonded to central metal

Charge on Complex Ion

Q_{co m pl e x} = Q_{m e t a l} + \sum Q_{l i g an d s}

Ligand Notation Priority (IUPAC)

alphabetical order in name; anionic prefix - o (chlorido, cyanido, sulfato)

Crystal Field Splitting — Octahedral★ Board fav

Δ_{o} = E (e_{g}) - E (t_{2 g})

Crystal Field Splitting — Tetrahedral★ Board fav

Δ_{t} = \frac{4}{9} Δ_{o}

Crystal Field Stabilization Energy (CFSE)

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

Magnetic Moment (spin only)★ Board fav

μ = n (n + 2) BM

Effective Atomic Number (EAN)

EAN = Z - Q_{o x} + 2 \times C N

Spectrochemical Series (ligand strength)

I^{-} < B r^{-} < C l^{-} < F^{-} < O H^{-} < H_{2} O < N H_{3} < e n < C N^{-} < C O

Low-Spin vs High-Spin Condition

High-spin: Δ_{o} < P; Low-spin: Δ_{o} > P

✏️ Solved Examples

1Solved Exampleeasy3 steps

Give the IUPAC name of K₃[Fe(CN)₆] and determine the oxidation state of Fe.

Identify the anion complex [Fe(CN)₆]³⁻.

6 CN^{-} ligands, complex charge = - 3

2Solved Exampleboard4 steps

Predict the hybridization, geometry and magnetic nature of [Co(NH₃)₆]³⁺. Given Co (Z = 27).

Oxidation state of Co: +3, so Co³⁺ = [Ar]3d⁶.

C o^{3 +} : 3 d^{6}

3Solved ExampleHOTS4 steps

For a d⁶ ion in an octahedral field, calculate CFSE (in units of Δ_o) for (a) high-spin, (b) low-spin cases. Ignore pairing energy.

High-spin d⁶: t2g⁴ eg².

CFSE = (- 0.4 \times 4 + 0.6 \times 2) Δ_{o} = - 0.4 Δ_{o}

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Naming cationic complexes with -ate suffix
✓-ate suffix is only for anionic complexes: e.g. ferrate, cuprate. Cationic keeps the metal name.
2
Assuming coordination number = oxidation state
✓CN is number of ligand atoms bonded; oxidation state is the charge on the metal. Different concepts.
3
Writing NH₃ as a strong-field ligand always
✓NH₃ is moderate to strong. CN⁻ and CO are strongest. Refer spectrochemical series.
4
Using Δ_t = Δ_o (not 4/9 Δ_o)
✓Tetrahedral field splitting is roughly 4/9 of octahedral; and t₂ (upper) > e (lower) in tetrahedral.
5
Applying spin-only formula μ = √(n(n+2)) with n = total electrons
✓n is number of UNPAIRED electrons only. Paired ones contribute zero to spin.
6
Confusing ionization isomerism with linkage isomerism
✓Ionization: swap ion inside/outside coord sphere. Linkage: same ligand bonds through different atoms (e.g. NO₂⁻ vs ONO⁻).

🎯 Practice Yourself

🎯Practice Yourself5 questions

Q1
Give the IUPAC name of [Co(NH₃)₅Cl]Cl₂.
Q2
Predict the geometry of [NiCl₄]²⁻ and its magnetic moment.
Q3
Which of [Fe(CN)₆]³⁻ and [Fe(H₂O)₆]³⁺ is high-spin?
Q4
State the postulates of Werner's theory.
Q5
Draw and name the two geometrical isomers of [Co(NH₃)₄Cl₂]⁺.

📝 Notes

Coordination Compounds — Maharashtra HSC Overview

Chapter 9 of the Balbharati Class 12 Chemistry textbook covers Werner's theory, IUPAC nomenclature, isomerism, VBT, CFT, and applications of coordination compounds.

Maharashtra syllabus specifics

Werner's postulates and derivation of primary/secondary valency for CoCl₃·6NH₃ series is a Maharashtra-specific 3-mark question. Balbharati devotes considerable text to this historical foundation.
VBT prediction of hybridization, geometry and magnetic behaviour — the "big three" for every complex — is asked every year.
Crystal Field Theory — octahedral splitting, spectrochemical series, high-spin vs low-spin — is treated with more rigour than at CBSE Class 12 level. Expect a HOTS numerical on CFSE.
Chelate effect and stability and application of complexes in biology (haemoglobin, chlorophyll) are Balbharati-specific 2-mark topics.

Board exam strategy

Master IUPAC nomenclature first — most compounds now use the "-ido" suffix (chlorido, not chloro). Balbharati follows the 2005 IUPAC rules.
For isomerism, mention TYPE (geometrical / optical / ionization / linkage / coordination / hydrate) and DRAW both isomers.
For magnetic moment: μ = √(n(n+2)) BM with n = unpaired electrons.
Distinguish weak-field vs strong-field ligands using the spectrochemical series — Balbharati provides the full ordering.