Match coordination compounds with their magnetic moments or geometry

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ChemistryCoordination Compounds

Match List I (Complex) with List II (Number of unpaired electrons):

List I (Complex)

A. [Fe(CN)₆]⁴⁻

B. [FeF₆]³⁻

C. [CoF₆]³⁻

D. [Co(NH₃)₆]³⁺

List II (Unpaired e⁻)

I. 0

II. 4

III. 5

IV. 4

Options

A–I, B–III, C–II, D–I

A–II, B–III, C–IV, D–I

A–I, B–II, C–III, D–IV

A–III, B–II, C–I, D–IV

Correct Answer

A–I, B–III, C–II, D–I (Option 1)

Solution — Unpaired Electrons

[Fe(CN)₆]⁴⁻ — Fe²⁺, strong field CN⁻ → 0 unpaired electrons

Fe²⁺: [Ar]3d⁶. CN⁻ is a strong field ligand → forces pairing → d⁶ low spin: all 6 electrons paired in 3 lower d-orbitals (t₂g⁶). Unpaired = 0 → diamagnetic ✓

[FeF₆]³⁻ — Fe³⁺, weak field F⁻ → 5 unpaired electrons

Fe³⁺: [Ar]3d⁵. F⁻ is a weak field ligand → no pairing forced → d⁵ high spin: all 5 d-orbitals singly occupied. Unpaired = 5 → μ = √(5×7) = √35 ≈ 5.92 BM ✓

[CoF₆]³⁻ — Co³⁺, weak field F⁻ → 4 unpaired electrons

Co³⁺: [Ar]3d⁶. F⁻ weak field → high spin d⁶: t₂g⁴eg² → 4 unpaired. Unpaired = 4 ✓

[Co(NH₃)₆]³⁺ — Co³⁺, strong field NH₃ → 0 unpaired electrons

Co³⁺: [Ar]3d⁶. NH₃ is a strong field ligand → low spin d⁶: t₂g⁶eg⁰ → all paired. Unpaired = 0 → diamagnetic ✓

Theory: Coordination Compounds

1. Crystal Field Theory — Strong and Weak Field Ligands

In an octahedral complex, the five d-orbitals split into two groups under the influence of ligand field: the lower t₂g set (dxy, dyz, dxz — three orbitals) and the higher eg set (dx²−y², dz² — two orbitals). The energy difference is Δo (crystal field splitting energy). Strong field ligands produce large Δo → electrons prefer to pair in lower t₂g orbitals (low spin). Weak field ligands produce small Δo → electrons spread out following Hund's rule (high spin).

μ = √(n(n+2)) BM, where n = number of unpaired electrons

Diamagnetic: n=0, μ=0 | Paramagnetic: n>0, μ>0

2. Spectrochemical Series

Weak field → Strong field (increasing Δo):

📌 I⁻ < Br⁻ < Cl⁻ < F⁻ < OH⁻ < ox²⁻ < H₂O < NCS⁻ < py < NH₃ < en < NO₂⁻ < CN⁻ < CO

📌 F⁻, Cl⁻, Br⁻, I⁻ = weak field (halides)

📌 NH₃, en = moderate field

📌 CN⁻, CO = strongest field ligands

3. IUPAC Nomenclature of Complexes

Rules: (1) Cation named first, then anion. (2) Within coordination sphere: ligands alphabetically, then central metal. (3) Anionic ligands end in -o (chloro, cyano, hydroxo). (4) Neutral ligands: aqua (H₂O), ammine (NH₃), carbonyl (CO), nitrosyl (NO). (5) Metal oxidation state in Roman numerals in parentheses. (6) Anionic complex: metal name ends in -ate (ferrate, cobaltate, cuprate).

4. Werner's Theory and Effective Atomic Number

Werner proposed coordination number and primary/secondary valence. The Effective Atomic Number (EAN) rule: central metal + electrons from ligands = EAN of next noble gas. EAN = Z + electrons donated by ligands. Many stable complexes satisfy EAN rule (18-electron rule): [Fe(CO)₅]: Fe(0) = 26e, 5CO donate 10e → 36 = Kr ✓. However, many stable complexes don't follow EAN rule (e.g., most ionic complexes).

5. Chelates and Stability

Chelating ligands bind to the metal through two or more donor atoms, forming ring structures. Examples: ethylenediamine (en) — bidentate; EDTA — hexadentate. Chelate complexes are more stable than corresponding non-chelate complexes (chelate effect — entropy increase on chelation). EDTA forms very stable complexes with almost all metal ions — used in water softening, metal ion determination, and as a preservative in food.

6. Isomerism in Coordination Compounds

📌 Geometrical: cis-trans in square planar (MA₂B₂) and octahedral (MA₄B₂)

📌 Optical: non-superimposable mirror images (chiral complexes)

📌 Linkage: ambidentate ligands — NO₂⁻ as nitro (N-bound) or nitrito (O-bound)

📌 Ionisation: [Co(SO₄)(NH₃)₅]Br vs [Co(Br)(NH₃)₅]SO₄

📌 Coordination: [Cu(NH₃)₄][PtCl₄] vs [Pt(NH₃)₄][CuCl₄]

📌 Solvate: [Cr(H₂O)₆]Cl₃ vs [CrCl(H₂O)₅]Cl₂·H₂O

7. Applications of Coordination Compounds

Coordination compounds have vast industrial and biological applications. Haemoglobin: Fe²⁺ complex with porphyrin ring — carries O₂. Chlorophyll: Mg²⁺ complex — photosynthesis. Vitamin B₁₂: Co complex. Cisplatin (cis-[PtCl₂(NH₃)₂]): anticancer drug — binds to DNA, preventing replication of cancer cells. EDTA: used in analytical chemistry to determine metal ion concentrations. Extraction of gold and silver: gold dissolves in NaCN solution due to complex formation: 4Au + 8CN⁻ + O₂ + 2H₂O → 4[Au(CN)₂]⁻ + 4OH⁻.

8. Magnetic Properties and Hybridisation

📌 Octahedral strong field (d⁶): t₂g⁶, 0 unpaired, sp³d² or d²sp³, diamagnetic

📌 Octahedral weak field (d⁶): t₂g⁴eg², 4 unpaired, sp³d², paramagnetic

📌 Octahedral (d⁵) strong field: t₂g⁵, 1 unpaired | weak field: t₂g³eg², 5 unpaired

📌 Tetrahedral: always high spin (small splitting), sp³

📌 Square planar: dsp² hybridisation, low spin d⁸ complexes (Ni²⁺, Pd²⁺, Pt²⁺)

Frequently Asked Questions

1. Why does CN⁻ cause pairing (low spin) but F⁻ does not? ⌄

CN⁻ is a strong field (π-acceptor) ligand — it creates a large crystal field splitting energy Δo. When Δo > pairing energy, electrons prefer to pair in lower orbitals. F⁻ is a weak field (σ-donor only) ligand with small Δo. When Δo < pairing energy, electrons spread out following Hund's rule (high spin). The spectrochemical series ranks ligands by Δo they cause.

2. How do you determine oxidation state of metal in [Fe(CN)₆]⁴⁻? ⌄

Overall charge = −4. CN⁻ ligand has charge −1. Six CN⁻ contribute: 6×(−1) = −6. Metal charge + (−6) = −4. Metal charge = −4 + 6 = +2. So Fe is +2 (Fe²⁺). Configuration: Fe²⁺ = [Ar]3d⁶. With strong field CN⁻: low spin d⁶, t₂g⁶, 0 unpaired electrons, diamagnetic.

3. What is the magnetic moment of [FeF₆]³⁻? ⌄

Fe³⁺ = 3d⁵. F⁻ weak field → high spin → all 5 d-orbitals singly occupied → 5 unpaired electrons. μ = √(n(n+2)) = √(5×7) = √35 ≈ 5.92 BM. This is the maximum possible magnetic moment for a d⁵ metal. Experimentally measured values close to this confirm 5 unpaired electrons.

4. What is the IUPAC name of [Co(NH₃)₆]³⁺? ⌄

Hexaamminecobalt(III) ion. Rules applied: ammine (NH₃ ligand), hexa (6 ligands), cobalt (central metal), (III) — oxidation state. If this were the cation in a compound with Cl⁻: hexaamminecobalt(III) chloride. Note: ammine (with double m) for NH₃ in complexes, not amine.

5. What is cisplatin and how does it work as anticancer drug? ⌄

Cisplatin = cis-[PtCl₂(NH₃)₂] — a square planar Pt(II) complex. The cis isomer is the active anticancer form (trans isomer is inactive). Inside the cell, Cl⁻ ligands are replaced by water (aquation), and the active complex then binds to N7 of adjacent guanine bases on the same DNA strand, forming an intrastrand crosslink. This distorts DNA, blocks replication, and triggers apoptosis in rapidly dividing cancer cells.

6. What is the chelate effect? ⌄

Chelate complexes are more stable than analogous complexes with monodentate ligands due to the chelate effect. Example: [Ni(en)₃]²⁺ is more stable than [Ni(NH₃)₆]²⁺ despite en and NH₃ having similar donor ability. The extra stability comes from increased entropy — when chelating ligand coordinates, fewer particles are in solution (ΔS positive). ΔG = ΔH − TΔS → more negative with positive ΔS → more stable chelate.

7. What are ambidentate ligands? Give examples. ⌄

Ambidentate ligands can bind through two different donor atoms. NO₂⁻ can bind through N (nitro-N, as in −NO₂ coordination) or through O (nitrito-O, as in −ONO coordination). SCN⁻ can bind through S (thiocyanato) or N (isothiocyanato). This leads to linkage isomerism — two complexes with same formula but ligand bound through different atoms. Example: [Co(NO₂)(NH₃)₅]²⁺ and [Co(ONO)(NH₃)₅]²⁺.

8. What is the role of coordination compounds in biological systems? ⌄

Haemoglobin: Fe²⁺ in porphyrin ring (haem) binds O₂ reversibly in lungs, releases it in tissues. CO poisoning: CO binds Fe²⁺ much more strongly than O₂ (200× greater affinity), preventing O₂ transport. Myoglobin: Fe²⁺ complex stores O₂ in muscles. Chlorophyll: Mg²⁺ porphyrin complex — absorbs light for photosynthesis. Vitamin B₁₂: Co complex — involved in DNA synthesis and nerve function. Carbonic anhydrase: Zn²⁺ enzyme — converts CO₂ to H₂CO₃ in blood.