Transition Elements (d-Block): Cr, Mn and Fe Groups – Properties, Trends, Oxidation States and Complex Formation For TGT, PGT, GATE, BITSAT, CSIR-NET, IIT-JAM, JEE Advanced, NEET, BHU Exams

Transition Elements: Groups 6, 7, and 8 — The Most Competitive Chemistry for JEE, NEET, GATE & CSIR-NET

Complete Chapter Coverage

Welcome to one of the most scoring and most feared chapters in inorganic chemistry. Groups 6, 7, and 8 — the Chromium, Manganese, and Iron groups — are not just textbook elements. They are the building blocks of civilization itself. Stainless steel, blood, batteries, catalysts, pigments, and even the nitrogen cycle all owe their existence to these remarkable metals. This article is your complete, exam-focused guide — written clearly, explained deeply, and packed with the kind of insight that helps you score in competitive exams.

Elements Covered: Cr, Mo, W (Group 6) | Mn, Tc, Re (Group 7) | Fe, Ru, Os (Group 8) — along with their oxides, halides, complexes, bioinorganic roles, and much more.

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Group 6 — The Chromium Group (Cr, Mo, W)

1. Electronic Configuration and Oxidation States

The first thing every examiner tests: why does chromium have an anomalous electronic configuration? Chromium (Cr) and molybdenum (Mo) both adopt a d⁵s¹ configuration instead of the expected d⁴s², because a half-filled d subshell provides exceptional stability due to symmetric electron distribution and exchange energy. Tungsten (W), however, adopts d⁴s² — one of the rare cases where the heavier element does not follow the same trend.

Element	Symbol	Configuration	Most Stable Oxidation States
Chromium	Cr	[Ar] 3d5 4s1	+III, +II, +VI (strongly oxidizing)
Molybdenum	Mo	[Kr] 4d5 5s1	+VI, +V, +IV, +III
Tungsten	W	[Xe] 4f14 5d4 6s2	+VI, +V, +IV

📌 Exam Trick: In Group 6, as you go DOWN the group, higher oxidation states become MORE stable. So Cr(+VI) is a powerful oxidizer, but Mo(+VI) and W(+VI) are stable. Conversely, Cr(+III) is stable while Mo(+III) and W(+III) are reducing. This is a classic "periodic trend in stability" question.

2. Occurrence and Extraction — Real World Chemistry

Chromium is the 21st most abundant element in the Earth's crust (~122 ppm). Its only commercially important ore is chromite, FeCr₂O₄ — a spinel mineral where O²⁻ ions are arranged in a cubic close-packed lattice, Fe²⁺ sits in tetrahedral holes, and Cr³⁺ in octahedral holes.

Step-by-Step: Extraction of Pure Chromium

1. Step 1 — Roasting with NaOH: Chromite is fused with NaOH in air to oxidize Cr(+III) to Cr(+VI):

   4 FeCr₂O₄ + 8 NaOH + 3½ O₂ → 4 Na₂[CrO₄] + 2 Fe₂O₃ + 4 H₂O    (at ~1100 °C)

2. Step 2 — Dissolution & Acidification: Soluble sodium chromate Na₂CrO₄ is leached out; the insoluble Fe₂O₃ remains. On acidification, orange sodium dichromate Na₂Cr₂O₇ precipitates (less soluble):

   2 CrO₄²⁻ + 2 H⁺ → Cr₂O₇²⁻ + H₂O

3. Step 3 — Reduction to Cr₂O₃:

   Na₂[Cr₂O₇] + 2C → Cr₂O₃ + Na₂CO₃ + CO

4. Step 4 — Thermite Reduction:

   Cr₂O₃ + 2 Al → 2 Cr + Al₂O₃    (Goldschmidt / thermite reaction)

For industrial use, ferrochrome (Fe + Cr alloy) is made directly by reducing chromite with carbon in an electric furnace:

FeCr₂O₄ + 4C → Fe + 2Cr + 4CO

🎯 JEE/NEET/GATE Checkpoint — Extraction of Mo & W Mo occurs as the mineral molybdenite MoS₂. It is roasted in air: 2 MoS₂ + 7 O₂ → 2 MoO₃ + 4 SO₂. MoO₃ is dissolved in dilute NH₄OH and reduced with H₂ to give pure Mo. W occurs as wolframite (FeWO₄·MnWO₄) and scheelite (CaWO₄). After acidification to give "tungstic acid" (WO₃·nH₂O), it is reduced with H₂ at 850 °C. Both metals come out as powders due to very high melting points (Mo: 2620 °C, W: 3380 °C — W has the second-highest melting point of all elements after carbon).

3. Oxidation States in Detail — The Most Tested Part

(a) Cr(+VI) State — Chromates and Dichromates

Cr(+VI) compounds are among the most powerful oxidizing agents known in analytical chemistry. They include chromate ion [CrO₄]²⁻, dichromate ion [Cr₂O₇]²⁻, chromium trioxide CrO₃, and oxohalides like CrO₂Cl₂.

The Chromate–Dichromate Equilibrium

This is one of the most important equilibria in inorganic chemistry:

2 CrO₄²⁻(yellow) + 2 H⁺ ⇌ Cr₂O₇²⁻(orange) + H₂O

CrO₄²⁻ (tetrahedral) — in basic medium

Cr₂O₇²⁻ (two tetrahedra sharing one corner O) — in acidic medium

Potassium Dichromate K₂Cr₂O₇ — The Volumetric Star

K₂Cr₂O₇ is preferred over Na₂Cr₂O₇ as a primary standard because the sodium salt is hygroscopic while the potassium salt is not. In acidic solution, it acts as a strong oxidizer with E° = +1.33 V:

½ Cr₂O₇²⁻ + 7H⁺ + 3e⁻ ⇌ Cr³⁺ + 3½ H₂O    E° = +1.33 V

⚠️ Health Warning (relevant in GATE/CSIR-NET too): Cr(+VI) compounds — dichromate, chromate — are carcinogenic. Ingestion or skin contact with K₂Cr₂O₇ or KCrO₄ during titrations requires careful handling.

(b) Chromium Trioxide CrO₃ — Chromic Acid

CrO₃ is a bright orange solid, commonly called "chromic acid." It is prepared by:

Na₂Cr₂O₇ + H₂SO₄ → 2 CrO₃ + Na₂SO₄ + H₂O

CrO₃ has a chain structure of fused tetrahedra. It is strongly acidic and dissolves in NaOH forming CrO₄²⁻. Above 250 °C it decomposes in stages:

2 CrO₃ → 2 CrO₂ + O₂

2 CrO₂ → Cr₂O₃ + ½ O₂

(c) Peroxo Compounds of Chromium — Deep Blue CrO(O₂)₂

When H₂O₂ is added to an acidified dichromate solution, a deep-blue peroxo compound forms:

Cr₂O₇²⁻ + 2H⁺ + 4H₂O₂ → 2 CrO(O₂)₂ + 5H₂O

This deep blue CrO(O₂)₂ decomposes quickly in aqueous solution to Cr³⁺ but can be stabilized by extraction into ether and reaction with pyridine (py) to form py·CrO(O₂)₂, which has a pentagonal pyramidal structure. This is a classic qualitative test for chromium!

🎯 JEE Advanced / CSIR-NET Must Know — Chromyl Chloride Test Chromyl chloride CrO₂Cl₂ is a deep red liquid used to confirm the presence of Cl⁻ in a sample:

K₂Cr₂O₇ + 6 HCl (conc. H₂SO₄) → 2 CrO₂Cl₂ + 2 KCl + 3 H₂O

CrO₂Cl₂ + 4 NaOH → Na₂CrO₄ + 2 NaCl + 2 H₂O (solution turns yellow)

Key: The deep red vapors when passed into NaOH give a yellow solution of Na₂CrO₄.

(d) Cr(+III) State — The Most Stable and Most Complex

Cr(+III) is the most stable and most important state for chromium. The Cr³⁺ ion has a d³ configuration with very high crystal field stabilization energy (CFSE), which makes its octahedral complexes extremely stable and kinetically inert.

• Cr₂O₃ — green solid, corundum (Al₂O₃) structure, used as a green pigment in paint, rubber, cement; catalyst in the Deacon process and in making polyethylene
• CrCl₃ — red-violet flakes (layer lattice), forms hexaaqua ion [Cr(H₂O)₆]³⁺ (violet) in solution; multiple halide-aqua complexes exist
• Chrome alum — KCr(SO₄)₂·12H₂O, a double salt: K[Cr(H₂O)₆][SO₄]₂

Cr(+III) Complexes: Isomerism Heaven for Exams

The Cr³⁺ ion forms numerous isomers with ammine and halide ligands. This is very heavily tested in JEE and CSIR-NET:

• [Cr(NH₃)₆]³⁺ — only one form (all equivalent NH₃)
• [Cr(NH₃)₅Cl]²⁺ — only one form
• [Cr(NH₃)₄Cl₂]⁺ — cis and trans isomers
• [Cr(NH₃)₃Cl₃] — mer and fac isomers
• [Cr(oxalate)₃]³⁻ — d and l optical isomers

Magnetic moment ≈ 3.87 BM (3 unpaired electrons, spin-only formula: μ = √(n(n+2)))

Electronic Spectrum of [Cr(H₂O)₆]³⁺

Cr(+III) is a d³ ion. In an octahedral crystal field, the ground state is ⁴A_2g(F). The three spin-allowed transitions are:

• ⁴A_2g(F) → ⁴T_2g(F) at ~17,400 cm⁻¹
• ⁴A_2g(F) → ⁴T_1g(F) at ~24,700 cm⁻¹
• ⁴A_2g(F) → ⁴T_1g(P) — charge transfer at ~37,800 cm⁻¹

(e) Cr(+II) State — The Quadruple Bond Marvel

Chromium(II) acetate dihydrate Cr₂(CH₃COO)₄·2H₂O is one of the most famous compounds in all of inorganic chemistry. Cr²⁺ has a d⁴ configuration with 4 unpaired electrons, but this compound is diamagnetic. Why? Because all four unpaired electrons participate in metal-metal bonding through a quadruple bond: one σ, two π, and one δ bond — giving a Cr–Cr distance of only 2.36 Å. Quadruple bonds are also found in Mo₂(CH₃COO)₄, Re₂Cl₈²⁻, and others.

4. MoO₃, WO₃ versus CrO₃ — Key Differences

Property	CrO3	MoO3	WO3
Colour	Orange	White (d0)	Lemon yellow
Solubility in H2O	Very soluble	Insoluble	Insoluble
Oxidizing power	Strong	Almost none	Almost none
Melting point	197 °C	795 °C	1473 °C
Structure	Chain of tetrahedra	Layer lattice	Distorted ReO3

5. Polyacids of Mo and W — Isopolyacids & Heteropolyacids

When molybdate [MoO₄]²⁻ or tungstate [WO₄]²⁻ solutions are acidified, the coordination number of Mo or W increases from 4 to 6 (by adding water molecules). This leads to a series of condensed anions — isopolyacids:

[MoO₄]⁴⁻ —pH 6—> [Mo₇O₂₄]⁶⁻ (paramolybdate) —pH 1-2—> [Mo₈O₂₆]⁴⁻ (octamolybdate) —pH<1—> MoO₃·2H₂O

🎯 CSIR-NET / GATE Checkpoint — Heteropolyacids A classic heteropolyacid test: A phosphate solution + ammonium molybdate + nitric acid → yellow precipitate of ammonium phosphomolybdate (NH₄)₃[PO₄·Mo₁₂O₃₆]. The 12-phosphomolybdic acid has 12 MoO₆ octahedra surrounding a central PO₄ tetrahedron. This test is used in gravimetric analysis for phosphate.

6. Tungsten Bronzes — A Special Mention

Tungsten bronzes are nonstoichiometric compounds of formula M_xWO₃ (where M = Na, K, or a lanthanide; x < 1). They are semi-metallic solids with a lustre and conduct electricity. Colour depends on x: x ≈ 0.9 (yellow/gold), x ≈ 0.5 (red), x ≈ 0.3 (blue-black). They are inert to strong acids and alkalis and used in "bronze" and "metallic" paints.

7. Biological Importance of Group 6

• Cr(+III) and insulin work together to maintain correct blood glucose levels. In Cr deficiency, glucose removal from blood is halved. Some cases of diabetes reflect faulty Cr metabolism.
• Mo is present in the enzyme nitrogenase — which fixes atmospheric N₂ biologically (175 million tonnes/year). The enzyme contains two proteins: molybdoferredoxin and azoferredoxin. It is thought that N₂ bonds to Mo through antibonding orbitals, and after reduction gives NH₃.

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Group 7 — The Manganese Group (Mn, Tc, Re)

1. Electronic Configuration and Oxidation States

All three elements have d⁵s² configurations, giving a maximum possible oxidation state of +VII. Manganese is unique in the entire periodic table for showing the widest range of oxidation states — from −III to +VII! The (+II) state is the most stable and most common for Mn because Mn²⁺ has a half-filled d⁵ configuration, giving maximum exchange energy stabilization.

Element	Symbol	Configuration	Most Stable States
Manganese	Mn	[Ar] 3d5 4s2	+II, +IV, +VII
Technetium	Tc	[Kr] 4d5 5s2	+VII, +IV
Rhenium	Re	[Xe] 4f14 5d5 6s2	+VII, +IV, +III

📌 Periodic Trend Alert: As you descend Group 7: Mn(+VII) is an extreme oxidizer (permanganate KMnO₄), but Tc(+VII) and Re(+VII) show only mild oxidizing properties. The (+II) and (+III) states are rare for Tc and Re — they prefer higher states. This is the opposite of what happens in Groups 4–6.

2. Technetium — The First Man-Made Element

Technetium does not occur in nature. All its isotopes are radioactive. ⁹⁹Tc is a β-emitter with half-life 2.1 × 10⁵ years. It is obtained in kilogram quantities from spent nuclear fuel rods as a fission product of uranium. Small amounts of ^99mTc compounds are injected into patients for radiographic scanning of organs (particularly the liver and thyroid). ⁹⁷Tc and ⁹⁸Tc can be made by neutron bombardment of Mo.

3. Extraction and Uses of Manganese

Manganese is the 12th most abundant element in Earth's crust (~1060 ppm). The chief ore is pyrolusite MnO₂. Pure Mn is obtained by electrolysis of aqueous MnSO₄ solutions. About 95% of all Mn mined is used in the steel industry:

• Ferromanganese (80% Mn) — made by reducing Fe₂O₃ + MnO₂ with C in a blast furnace. Used as a scavenger (removes O and S, preventing bubbles and brittleness) and for making Hadfield steel (~13% Mn, 1.25% C — extremely hard and shock-resistant, used in rock-crushing machinery)
• Silicomanganese (~65% Mn, 20% Si, 15% Fe)
• Spiegeleisen (5–25% Mn, similar to cast iron)
• Manganin (84% Cu, 12% Mn, 4% Ni) — electrical resistance almost unaffected by temperature; used in precision electrical instruments

4. Oxidation States in Detail

(a) Mn(+II) — The Pale Pink d⁵ King

Mn(+II) salts are the most common. The [Mn(H₂O)₆]²⁺ ion is pale pink. The d⁵ configuration gives zero CFSE in an octahedral field (all five d orbitals are singly occupied, symmetrically) — so complexes like [Mn(NH₃)₆]²⁺ and [MnCl₆]⁴⁻ are not stable in isolation. Chelate ligands like EDTA form more stable complexes.

Why are Mn(+II) complexes so pale? Because with d⁵ high-spin configuration, any d–d electronic transition requires not only moving an electron from t_2g to e_g, but also reversing its spin. This is spin-forbidden (violates the spin selection rule: ΔS ≠ 0). Spin-forbidden transitions have very low probability → very pale colours (colour intensity about 1/100 of normally allowed transitions).

Mn(+II) in alkaline solution is easily oxidized. Addition of NaOH to Mn²⁺ solution gives pale pink gelatinous Mn(OH)₂ precipitate, which rapidly turns brown-black as it oxidizes to MnO₂:

Mn(OH)₂ + ½ O₂ → MnO₂ + H₂O    (in alkaline solution, easily)

(In acid solution: Mn²⁺ is stable, E°(Mn³⁺/Mn²⁺) = +1.51 V — hard to oxidize in acid)

(b) Mn(+III) — The Unusual Disproportionator

Mn³⁺ has d⁴ configuration. Only a small number of Mn(+III) salts are known — MnF₃, Mn₂(SO₄)₃, and Mn₂O₃. The hydrated [Mn(H₂O)₆]³⁺ ion cannot be obtained in concentrated solutions because it disproportionates in acid:

2 Mn³⁺ + 2 H₂O → Mn²⁺ + MnO₂ + 4 H⁺    (acid)

The Jahn-Teller theorem predicts distortion of Mn(+III) octahedral complexes (electronic structure (t_2g)³(e_g)¹ — e_g not symmetrically filled), similar to Cu²⁺. Manganese acetate [Mn₃O(CH₃COO)₆L₃]⁺ (basic acetate, same structure as Cr and Fe basic acetates) is used industrially to oxidize toluene to benzaldehyde.

(c) Mn(+IV) — The Battery Mineral MnO₂

MnO₂ is the most important Mn(+IV) compound. It occurs naturally as pyrolusite (the main Mn ore) and has the rutile (TiO₂) structure. Key uses:

• Dry batteries (Leclanché cells) — over half a million tonnes used annually for 'dry batteries'. Must be very pure and is made electrolytically.
• Brick industry — colours bricks red or brown.
• Glass industry — makes red or purple glass.
• Organic chemistry — oxidizes activated alcohols and alkenes to aldehydes/ketones.
• Catalyst — decomposes KClO₃ at 150 °C (instead of 400–500 °C without catalyst):

2 KClO₃ —MnO₂, 150°C→ 2 KCl + 3 O₂

MnO₂ in concentrated HCl is reduced and Cl₂ is liberated (Scheele's method — the original industrial method for Cl₂ before electrolysis):

MnO₂ + 4 HCl → MnCl₂ + Cl₂ + 2 H₂O

(d) Mn(+VII) — Permanganate: The Oxidizing Legend

KMnO₄ is intensely purple-black and is one of the most important oxidizing agents in chemistry. The Mn is in a d⁰ configuration, so the intense colour comes from charge transfer spectra (O²⁻ → Mn⁷⁺), not d–d transitions.

🎯 JEE/NEET Must-Know — Industrial Preparation of KMnO₄

Step 1: MnO₂ + KNO₃ —fuse with NaOH—> K₂MnO₄ + NO (dark green manganate)

Step 2: K₂MnO₄ + H₂O —electrolytic oxidation—> KMnO₄ + KOH + ½ H₂

Key: In dilute alkali MnO₄²⁻ (green) disproportionates: 3 MnO₄²⁻ + 2H₂O → 2 MnO₄⁻ + MnO₂ + 4 OH⁻

Important Reactions of KMnO₄

• Oxidizes H₂S → S (in acid)
• Oxidizes Fe²⁺ → Fe³⁺ (in acid) — used in volumetric analysis
• Oxidizes oxalate (C₂O₄²⁻) → CO₂ (in hot acid) — Mohr's salt titration
• Oxidizes toluene → benzoic acid; benzaldehyde; oxidizes alkenes to diols
• Disinfects drinking water (oxidizes & kills bacteria without Cl₂ taste)
• Manufacture of saccharin, ascorbic acid (Vitamin C), nicotinic acid (niacin)

(e) Mn(+VI) — Dark Green Manganate

Only one Mn(+VI) compound: the manganate ion MnO₄²⁻. These dark green compounds are formed by fusing MnO₂ in KOH with air or KNO₃. They are unstable in dilute alkali and disproportionate (as shown above). They are stable only in concentrated alkali.

5. Tc and Re — The Upper Row Peculiarities

Technetium and rhenium strongly resemble each other (due to the lanthanide contraction). They differ from Mn in:

• Almost no aqueous ionic chemistry except TcO₄⁻ and ReO₄⁻ (the only stable ions in water)
• Strong tendency to form metal–metal bonds in lower oxidation states (+II, +III, +IV)
• Tc(+VII) and Re(+VII) are only slightly oxidizing (unlike Mn(+VII))
• Re has the second highest melting point of all metals (3180 °C; W is highest at 3380 °C)

The Quadruple Bond in [Re₂Cl₈]²⁻

When perrhenates are reduced by H₂ or NaH₂PO₂ in HCl, the remarkable ion [Re₂Cl₈]²⁻ forms. The Re–Re bond length is only 2.24 Å — interpreted as a quadruple bond: 1σ + 2π + 1δ. The two ReX₄ planes are eclipsed (0°), which is a direct consequence of the δ bond (d_xy–d_xy overlap). If the δ bond were absent, the planes would be staggered.

6. Biological Role of Manganese

• Mn^II is essential in plant and animal enzymes. In mammals, the enzyme arginase (produced in the liver) converts nitrogenous waste to urea via the ornithine–arginine–citrulline cycle (discovered by Hans Krebs).
• Mn is an essential trace element for plant growth — added to fertilizers in deficient soils.
• Mn(+III) and Mn(+IV) complexes are involved in the release of O₂ during photosynthesis (the oxygen-evolving complex in photosystem II contains a Mn₄Ca cluster).

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Group 8 — The Iron Group (Fe, Ru, Os)

1. Introduction: Iron — The Most Important Metal

Iron is used in larger quantities than any other metal. It is the fourth most abundant element in Earth's crust (~62,000 ppm, or 6.2% by weight). Biologically, it is the most important transition element — involved in oxygen transport (haemoglobin), oxygen storage (myoglobin), electron transfer (cytochromes, ferredoxins), and nitrogen fixation (nitrogenase). Fe forms unusual organometallic compounds, the most famous being ferrocene.

2. Electronic Configuration and Oxidation States

Element	Symbol	Configuration	Most Stable States
Iron	Fe	[Ar] 3d6 4s2	+II, +III
Ruthenium	Ru	[Kr] 4d7 5s1	+III, +IV, +VIII
Osmium	Os	[Xe] 4f14 5d6 6s2	+IV, +VIII

📌 Key Trend Alert: In Group 8, the maximum oxidation state for Fe is only +VI (rare) — the trend of increasing the maximum oxidation state to +VIII only holds for Ru and Os (which form RuO₄ and OsO₄ — tetroxides in the +VIII state). Fe does NOT form FeO₄ as a stable compound. OsO₄ is the second-most dense element (22.57 g cm⁻³), exceeded only by Ir.

3. Extraction of Iron — Blast Furnace Chemistry

The blast furnace is one of the most tested topics in competitive exams. Let's do it step by step:

Raw Materials Charged from the Top:

1. Iron ore (Fe₂O₃ — haematite, Fe₃O₄ — magnetite, FeO(OH) — limonite)
2. Coke (reducing agent + fuel)
3. Limestone CaCO₃ (slag-forming agent)

Temperature-Dependent Reactions in the Blast Furnace:

200–400 °C: 3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂

400–700 °C: Fe₃O₄ + CO → 3FeO + CO₂

500–600 °C: 2CO → C + CO₂ (C deposits as soot, reduces FeO)

700 °C: FeO + C → Fe + CO (also FeO + CO → Fe + CO₂)

900 °C: CaCO₃ → CaO + CO₂

1000 °C: FeO + CO → Fe + CO₂ | CO₂ + C → 2CO

1800 °C: CaO + SiO₂ → CaSiO₃ (slag) | MnO + C → Mn(in Fe) + CO

Overall reaction: 3C + Fe₂O₃ → 4Fe + 3CO₂ (simplified)
Molten iron (pig iron) collects at the bottom. It contains 3–4% C, 1–2% Si, 0.05–2% P, 0.05–1% S, 0.5–2% Mn. This brittleness from excess C is removed in steel making.

Steel Making — Know all Four Processes:

1. Puddling (obsolete): Mixing pig iron with Fe₃O₄ and burning off C → wrought iron
2. Bessemer process: Silica-lined converter; compressed air blown through molten pig iron burns Si, Mn, then C. Cannot remove P.
3. Thomas–Gilchrist process: Basic-lined (dolomite/limestone) converter for P-rich ores; basic slag (Ca₃(PO₄)₂) is a valuable phosphate fertilizer
4. Basic Oxygen Process (BOP) — Modern standard: pure O₂ blown at high speed onto molten pig iron through retractable water-cooled lances. Converts 300 tonnes in 40 minutes. No nitrogen uptake. Cleaner product.

🎯 JEE/NEET Checkpoint — Types of Steel by C% Mild steel: 0.15–0.3% C (malleable, ductile, most common) | Medium: 0.3–0.6% | High-carbon: 0.6–0.8% | Tool steel: 0.8–1.4% C (hardest). Stainless steel: 12–15% Ni, 10% Cr. High-speed cutting steel: 18% W, 5% Cr.

4. Fe(+II) State — Ferrous Chemistry

Fe(+II) salts are called ferrous salts. The [Fe(H₂O)₆]²⁺ ion is pale green. Fe²⁺ has a d⁶ configuration.

• FeSO₄·7H₂O (green vitriol) — pale green crystals, used as a mordant in dyeing and ink-making
• Mohr's salt — FeSO₄·(NH₄)₂SO₄·6H₂O — more stable than FeSO₄ alone (less easily oxidized), primary standard in volumetric analysis with dichromate and permanganate
• Fenton's reagent — FeSO₄ + H₂O₂ → generates hydroxyl radicals (OH•), used to oxidize alcohols to aldehydes
• FeO — nonstoichiometric, formula Fe_0.95O; NaCl structure; formed by heating Fe(II) oxalate in vacuum

High-Spin vs Low-Spin Fe²⁺ Complexes:

Fe²⁺ (d⁶) complexes can be either high-spin or low-spin depending on the ligand field strength:

Prussian Blue and Turnbull's Blue

The hexacyanoferrate(II) ion [Fe(CN)₆]⁴⁻ (yellow, ferrocyanide) and hexacyanoferrate(III) [Fe(CN)₆]³⁻ (orange-red, ferricyanide) form the most famous pigment pair:

• Prussian blue: KFe^III[Fe^II(CN)₆] — deep blue; formed when Fe³⁺ is added to ferrocyanide K₄[Fe(CN)₆]
• Turnbull's blue: KFe^II[Fe^III(CN)₆] — identical to Prussian blue by X-ray; formed when Fe²⁺ is added to ferricyanide K₃[Fe(CN)₆]
• White precipitate K₂Fe^II[Fe^II(CN)₆] forms when Fe²⁺ + ferrocyanide → test for Fe²⁺

Brown Ring Test for Nitrates

FeSO₄ (fresh) + NO₃⁻ + H₂SO₄(conc.) → [Fe(H₂O)₅NO]²⁺ (brown ring at interface)

The complex formally contains Fe(+I) and NO⁺; magnetic moment ≈ 3.9 BM (3 unpaired e⁻)

5. Fe(+III) State — Ferric Chemistry

Fe³⁺ has a d⁵ configuration — similar to Mn²⁺. Fe(+III) solutions are frequently yellow-brown due to colloidal iron oxide or FeO·OH. Fe(+III) forms complexes preferring O-donor ligands over N-donor ligands.

• Fe₂O₃ — haematite structure; red-brown; used as a pigment
• Fe₃O₄ — magnetite, a mixed oxide Fe^IIFe₂^IIIO₄ (inverse spinel), ferromagnetic; black solid formed at 1400 °C from Fe₂O₃
• FeCl₃ — black solid in gas phase (FeCl₃ dimer), layer lattice in solid; yellow-brown in water; used in manufacture of CCl₄, as mordant in dyeing

SCN⁻ Test for Fe(+III):

Fe³⁺ + SCN⁻ → [Fe(SCN)(H₂O)₅]²⁺ (blood-red colour)

→ further SCN⁻: [Fe(SCN)₂]⁺, Fe(SCN)₃, [Fe(SCN)₄]⁻, [Fe(SCN)₆]³⁻

Colour destroyed by F⁻ ions (forms [FeF₆]³⁻)

6. Fe(+VI) — Ferrate Ions

Ferrate(VI) ions FeO₄²⁻ are prepared by passing Cl₂ into alkaline hydrated iron(III) oxide, or by oxidizing Fe(+III) with NaOCl in alkali. They are purple and are stronger oxidizers than KMnO₄. Stability of +VI state across the period: CrO₄²⁻ > MnO₄²⁻ > FeO₄²⁻ >> CoO₄²⁻.

7. Ru and Os — The Noble Platinum Group

Ru and Os are very rare, noble, and resistant to attack. They are found in the metallic state with platinum metals. The most important compounds are:

• RuO₄ and OsO₄ — yellow volatile solids (tetrahedral, d⁰). OsO₄ is a biological stain (reduces to OsO₂ in tissue), used in organic chemistry to add to double bonds giving cis-diols. Both are toxic. OsO₄ is more stable than RuO₄.
• Ru(+II) NO complexes — an interesting series [Ru^IINO·L₄]ⁿ, where the NO ligand is strongly bonded through σ and π; the Ru–N–O angle can be linear or bent.
• Both Ru and Os form carbonyls: Os(CO)₅, Ru(CO)₅, and trinuclear Ru₃(CO)₁₂, Os₃(CO)₁₂. The dinitrogen complex [Ru(NH₃)₅N₂]Cl₂ was the first reported dinitrogen complex (1965).

8. Bioinorganic Chemistry of Iron — The Most Important Section

Haemoglobin — Nature's Oxygen Transporter

The human body contains about 4 g of iron. About 70% is in haemoglobin (red blood cells). Haemoglobin (Hb) has molecular weight ~65,000 and consists of four subunits, each containing a haem b group — a porphyrin ring with Fe²⁺ bonded to four N atoms and a fifth N from histidine in the globin protein. The sixth coordination position is either a water molecule (deoxy) or dioxygen (oxy).

• In oxyhaemoglobin: Fe²⁺ is low-spin (diamagnetic) — it fits in the porphyrin plane. O₂ occupies the sixth position.
• In deoxyhaemoglobin: Fe²⁺ is high-spin (paramagnetic, 4 unpaired e⁻) and larger (radius increases by 28%) — it sits 0.7–0.8 Å above the porphyrin ring, distorting the complex.
• Cooperative effect: When one subunit picks up O₂, Fe²⁺ contracts and moves into the plane → pulls histidine → causes conformational changes in other subunits → enhances their ability to attract O₂.
• CO poisoning: CO binds irreversibly to the same site as O₂, preventing oxygen transport → lethal.

Myoglobin

Similar to one haemoglobin subunit; only one Fe atom; MW ~17,000. Stores O₂ in muscle tissue and binds O₂ more strongly than haemoglobin.

Cytochromes

Electron carriers in the mitochondria. They contain Fe in both +II and +III states and cycle reversibly between them. There are three classes: cytochrome a (E° = +0.28 V), cytochrome b (E° = +0.04 V), cytochrome c (E° = +0.26 V). The sequence of electron transfer is b → c → a, gradually releasing energy stored as ATP.

Ferredoxins

Non-haem iron proteins responsible for electron transfer in plants and bacteria. They contain Fe–S clusters (Fe₂S₂ or Fe₄S₄ cubane-type). Rubredoxin (simplest) contains one Fe atom surrounded by 4 S atoms from cysteine residues.

Ferritin — Iron Storage

Animals absorb Fe as Fe(+II) from food. It immediately reacts to form transferrin (a transport protein with MW 76,000–80,000 carrying 2 Fe atoms per molecule). Fe is stored as ferritin — a roughly spherical protein shell (apoferritin, ~120 Å diameter) enclosing a micelle of Fe(+III) hydroxide/oxide/phosphate containing 2,000–5,000 Fe atoms.

9. Ferrocene and Organometallic Chemistry

In 1951, G. Wilkinson and E.O. Fischer independently prepared an astonishing compound — ferrocene (π-C₅H₅)₂Fe — marking the birth of modern organometallic chemistry. For this work they were jointly awarded the Nobel Prize in Chemistry in 1973.

2 C₅H₅MgBr + FeCl₂ → Fe(C₅H₅)₂ + MgBr₂ + MgCl₂    (orange crystals)

2 C₅H₅⁻ + FeCl₂ → (η⁵-C₅H₅)₂Fe + 2 Cl⁻

• Ferrocene has a sandwich structure: Fe atom sandwiched between two parallel, planar cyclopentadienyl rings.
• Stable in air, thermally stable to 500 °C, soluble in organic solvents, insoluble in water and NaOH — orange crystals.
• The two C₅H₅ rings are in staggered conformation in the crystal (space group constraint), but with only ~4 kJ/mol barrier to rotation.
• Fe–C bond length: 2.06 Å. C–C bond in ring: 1.39 Å (same as benzene! — aromatic character).
• Ferrocene undergoes Friedel-Crafts acylation (electrophilic aromatic substitution) — e.g., acetylation with CH₃COCl in presence of AlCl₃.
• Rutenocene and osmocene are analogous compounds where the rings adopt the eclipsed conformation.

10. Iron Carbonyls

Fe(CO)₅ (iron pentacarbonyl) is a liquid at room temperature, available commercially. In carbonyls, CO acts as a σ donor and strong π acceptor (dπ–pπ* back bonding):

• Fe(CO)₅ — mononuclear, D_3h (trigonal bipyramidal)
• Fe₂(CO)₉ — dinuclear with three bridging CO groups, two terminal CO on each Fe
• Fe₃(CO)₁₂ — trinuclear; has two CO bridges between one pair of Fe atoms

Fe(CO)₅ + 3 NaOH → Na[HFe(CO)₄] + Na₂CO₃ + H₂O

Fe(CO)₅ + PF₃ → (PF₃)₃Fe(CO)₂ + 3 CO (substitution)

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Standard Reduction Potentials — Latimer Diagrams Summary

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🎯 Consolidated Exam Tips — JEE, NEET, GATE, CSIR-NET, IIT-JAM, BITSAT

Most Frequently Asked Topics (by Exam)

Topic	JEE Adv.	NEET	GATE/CSIR	IIT-JAM
Electronic configurations (Cr, Mo)	★★★	★★★	★★	★★★
K₂Cr₂O₇ reactions + titrations	★★★	★★★	★★★	★★★
KMnO₄ reactions (pH dependent)	★★★	★★★	★★★	★★★
Chromate–dichromate equilibrium	★★★	★★	★★★	★★★
Chromyl chloride test	★★	★★	★★	★★
Blast furnace reactions	★★	★★★	★★	★★
Prussian blue / ferrocyanide	★★★	★★	★★★	★★★
Haemoglobin / spin state of Fe²⁺	★★	★★★	★★★	★★★
Quadruple bonds (Cr₂, Re₂Cl₈²⁻)	★★	★	★★★	★★★
Ferrocene synthesis & bonding	★★	★	★★★	★★★
Crystal field theory (Mn²⁺, Fe³⁺)	★★★	★★	★★★	★★★
Isopolyacids / heteropolyacids (Mo, W)	★	★	★★★	★★★

Quick Revision — 30 Key Facts to Never Forget

1. Cr has [Ar] 3d⁵ 4s¹ (half-filled d is extra stable) — anomalous configuration.
2. Cr(+III) is the most stable state of Cr; Cr(+VI) is strongly oxidizing (E° = +1.33 V vs NHE).
3. Chromate is yellow in basic, dichromate is orange in acidic medium.
4. K₂Cr₂O₇ (not Na₂Cr₂O₇) is a primary standard — Na salt is hygroscopic.
5. CrO₃ is called "chromic acid" — bright orange, chain structure, acidic oxide.
6. Deep blue CrO(O₂)₂ is a qualitative test for Cr (chromate + H₂O₂ + acid).
7. Chromyl chloride test confirms Cl⁻ using K₂Cr₂O₇ + conc. H₂SO₄ → CrO₂Cl₂ (deep red vapour).
8. Cr(+II) acetate is diamagnetic due to a Cr≡Cr quadruple bond (σ + 2π + δ).
9. W has the second-highest melting point of all elements (3380 °C) after carbon.
10. MoS₂ is an excellent solid lubricant (layer lattice).
11. Mn has the widest range of oxidation states: −III to +VII.
12. Mn(+II) has d⁵ configuration — half-filled d → very stable; pale pink colour (spin-forbidden d–d).
13. Mn(+III) disproportionates in acid into Mn²⁺ + MnO₂.
14. MnO₂ (pyrolusite) — rutile structure; used in dry batteries, catalyst in O₂ generation.
15. KMnO₄ in acid gives Mn²⁺ (Δ = 5e⁻), in alkaline gives MnO₂ (Δ = 3e⁻).
16. KMnO₄ colour arises from charge transfer (d⁰), not d–d spectra.
17. Technetium is the first man-made element — all isotopes radioactive.
18. ^99mTc used in nuclear medicine (liver and organ scanning).
19. Fe is the fourth most abundant element in Earth's crust (62,000 ppm).
20. Fe oxidation states: +II (ferrous) and +III (ferric) most common; +VIII is for Os, Ru (tetroxides).
21. Fe²⁺ high-spin (weak field): 4 unpaired e⁻; low-spin (strong field, CN⁻): 0 unpaired e⁻.
22. Prussian blue = KFe^III[Fe^II(CN)₆] = Turnbull's blue (confirmed identical by X-ray).
23. Brown ring test: [Fe(H₂O)₅NO]²⁺ — test for nitrates.
24. SCN⁻ test gives blood-red colour with Fe³⁺ (destroyed by F⁻).
25. In haemoglobin, oxygenated Fe²⁺ is low-spin (diamagnetic); deoxygenated is high-spin (paramagnetic).
26. Cooperative effect in Hb: binding one O₂ enhances affinity of the other three subunits.
27. Cytochromes: electron carriers between +II and +III states; sequence b → c → a releases energy as ATP.
28. Ferrocene (Nobel Prize 1973, Fischer & Wilkinson): sandwich structure η⁵-C₅H₅ rings; undergoes Friedel-Crafts acylation.
29. OsO₄ is more stable than RuO₄; used in organic chemistry to add to double bonds (cis-diols).
30. Fe₃O₄ is an inverse spinel: Fe^IIFe₂^IIIO₄ — ferromagnetic, black solid.

🎯 BITSAT / JEE Main Speed Round — Fill in the Blank (Common MCQ Patterns)

• The ore of chromium is _____ (chromite, FeCr₂O₄)
• _____ is used as a primary standard in redox titrations (K₂Cr₂O₇)
• The colour of KMnO₄ is due to _____ (charge transfer, not d–d)
• MnO₄⁻ in acid medium accepts _____ electrons (5)
• Ferrocene was first synthesised in _____ (1951)
• The spin-only magnetic moment of [Fe(CN)₆]⁴⁻ is _____ BM (0 — diamagnetic)
• The ore of tungsten used in India is _____ (wolframite / scheelite)
• _____ is called "fool's gold" (FeS₂, iron pyrites)

Common Mistakes to Avoid

• Do NOT say Cr has d⁴s² configuration — it is d⁵s¹.
• Do NOT confuse chromate (yellow, basic) with dichromate (orange, acidic) — many students swap these.
• Do NOT say Prussian blue and Turnbull's blue are different compounds — they are identical.
• Do NOT say the colour of MnO₄⁻ is d–d — it's a d⁰ ion; colour is from charge transfer.
• Do NOT apply the blast furnace equation as a single step — it proceeds in stages at different temperatures.
• Do NOT forget that KMnO₄ titrations in acid give Mn²⁺ (5e⁻ change), while in alkaline give MnO₂ (3e⁻ change) — stoichiometry will be completely different.

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References & Further Reading: J.D. Lee, Concise Inorganic Chemistry, 5th Ed., Blackwell Science; Atkins, Shriver & Atkins' Inorganic Chemistry; IUPAC Recommendations for Nomenclature of Inorganic Chemistry (2005). All molecular geometries, bond angles, and oxidation states are consistent with IUPAC guidelines and experimental crystallographic data.

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