Inorganic Chemistry-4 (Complexes and Organometallics) BCH5T11

Syllabus topics (RTMNU)

• Limitations of Valency bond theory
• Crystal field theory: Splitting of d-orbital in octahedral, tetrahedral and square planar complexes
• Factors affecting the Magnitude of 10Dq
• Crystal Field Stabilisation Energy of Octahedral and Tetrahedral complexes, Numericals
• Jahn Teller Effect
• Selection Rules (Laporte and Spin selection Rules)
• Hole formalism principle
• Electronic spectrum of [Ti(H2O)6]3+ and [Cu(H2O)6]2+ complex ions

Limitations of Valence Bond Theory

Valence bond theory (Pauling) explains the shape and magnetic nature of complexes through hybridisation (inner-orbital d²sp³ and outer-orbital sp³d²), but it has many limitations:

1. It cannot explain the colour and electronic spectra of complexes.
2. It does not explain why some ligands are strong (cause pairing) and others weak.
3. It cannot explain the orbital contribution to magnetic moment or its variation with temperature.
4. It gives no quantitative idea of the energies or stability of complexes.
5. It cannot explain distortions such as the Jahn–Teller effect, or why Cu(II) square planar complexes use a 4p orbital (promotion of an electron).
6. The distinction between inner- and outer-orbital complexes is often arbitrary.

Crystal Field Theory: Splitting of d-Orbitals

Crystal field theory (CFT), given by Bethe and Van Vleck, treats ligands as point negative charges (or dipoles) and the metal–ligand bond as purely electrostatic. In a free metal ion the five d-orbitals are degenerate (equal energy). When ligands approach, electrons in d-orbitals pointing towards the ligands are repelled more, so the d-orbitals split into groups of different energy. The energy gap is the crystal field splitting energy, Δ or 10Dq.

Octahedral complexes: six ligands lie along ±x, ±y, ±z axes. The e_g set (d_x²−y², d_z²), pointing at ligands, is raised by +0.6Δ_o (+6Dq); the t_2g set (d_xy, d_yz, d_xz), lying between axes, is lowered by −0.4Δ_o (−4Dq). The barycentre (average energy) stays the same.

Tetrahedral complexes: four ligands approach between the axes, so the t₂ set (d_xy, d_yz, d_xz) is raised by +0.4Δ_t and the e set (d_x²−y², d_z²) is lowered by −0.6Δ_t — the reverse of octahedral. Δ_t = (4/9)Δ_o, so tetrahedral complexes are almost always high-spin.

Square planar complexes: obtained by removing the two z-axis ligands from an octahedron. Orbitals with a z-component fall in energy. Order: d_x²−y² (highest) > d_xy > d_z² > d_xz = d_yz (lowest). The large gap below d_x²−y² makes d⁸ ions (Ni²⁺ with strong ligands, Pd²⁺, Pt²⁺) square planar and diamagnetic, e.g. [Ni(CN)₄]²⁻.

Factors Affecting the Magnitude of 10Dq

1. Nature of the ligand: the spectrochemical series (increasing Δ): I⁻ < Br⁻ < S²⁻ < SCN⁻ < Cl⁻ < F⁻ < OH⁻ < C₂O₄²⁻ < H₂O < NCS⁻ < NH₃ < en < bipy < phen < NO₂⁻ < CN⁻ < CO. Ligands at the left are weak-field; at the right, strong-field.
2. Oxidation state of the metal: higher charge pulls ligands closer → larger Δ; e.g. Δ_o for [Co(H₂O)₆]³⁺ is much larger than for [Co(H₂O)₆]²⁺.
3. Nature of the metal (row): Δ increases by about 30–50% from 3d to 4d and again to 5d, so 4d/5d complexes are usually low-spin.
4. Geometry: Δ_t ≈ 4/9 Δ_o; square planar Δ is largest.
5. Number of ligands and metal–ligand distance (shorter distance → larger Δ).

Crystal Field Stabilisation Energy (CFSE) and Numericals

CFSE is the net lowering of energy of the d-electrons in a crystal field compared with the hypothetical spherical field. Whether electrons pair up depends on Δ compared with the pairing energy P: if Δ_o < P → high spin (weak field); if Δ_o > P → low spin (strong field). This choice exists only for d⁴–d⁷ octahedral ions.

Numerical: [Ti(H₂O)₆]³⁺ absorbs at 20,300 cm⁻¹. This equals Δ_o. Using 1 cm⁻¹ = 11.96 J mol⁻¹: Δ_o = 20,300 × 11.96 = 242.8 kJ mol⁻¹. CFSE = −0.4 × 242.8 = −97.1 kJ mol⁻¹ (or −8,120 cm⁻¹).

Jahn–Teller Effect

Jahn–Teller theorem (1937): "Any non-linear molecule in a degenerate electronic state will undergo a distortion to remove the degeneracy and lower its energy." In octahedral complexes, unequal filling of the e_g orbitals causes strong distortion — usually tetragonal elongation (two axial bonds longer, four equatorial shorter).

Strong Jahn–Teller distortion occurs for d⁹ (Cu²⁺), high-spin d⁴ (Cr²⁺, Mn³⁺) and low-spin d⁷. Weak distortion occurs with unequal t_2g filling (d¹, d²). No distortion for d³, high-spin d⁵, low-spin d⁶, d⁸, d¹⁰. Example: in [Cu(H₂O)₆]²⁺ (t_2g⁶e_g³), the two axial Cu–O bonds are longer than the four equatorial ones.

Selection Rules: Laporte and Spin

1. Laporte (orbital) rule: allowed transitions involve a change in parity (Δl = ±1, g ↔ u). d–d transitions (g → g) are Laporte-forbidden in centrosymmetric (octahedral) complexes. They still appear weakly (ε ≈ 1–10 L mol⁻¹ cm⁻¹) because molecular vibrations momentarily destroy the centre of symmetry (vibronic coupling). Tetrahedral complexes have no centre of symmetry, so their bands are more intense (ε ≈ 100–1000) — e.g. deep blue [CoCl₄]²⁻ versus pale pink [Co(H₂O)₆]²⁺.
2. Spin selection rule: ΔS = 0 — the number of unpaired electrons must not change. Spin-forbidden transitions are very weak; high-spin Mn²⁺ (d⁵) complexes have only spin-forbidden d–d bands and are almost colourless (pale pink).

Hole formalism: a dⁿ configuration can be treated as (10 − n) "holes" (positive charges) in a filled d-shell. The holes behave like electrons but with the energy order reversed. So d⁹ (one hole) behaves like d¹, d⁸ like d²; both d¹ and d⁹ octahedral ions give a single d–d band (²T_2g ↔ ²E_g, inverted for d⁹).

Electronic Spectra of [Ti(H₂O)₆]³⁺ and [Cu(H₂O)₆]²⁺

[Ti(H₂O)₆]³⁺ (d¹): ground state t_2g¹ (²T_2g). One electron is promoted to e_g (²T_2g → ²E_g), giving one broad band at about 20,300 cm⁻¹ (≈ 493 nm). It absorbs yellow-green light, so the solution looks purple/violet. The band is broad and has a shoulder because the excited state (e_g¹) undergoes Jahn–Teller distortion, splitting the e_g level.

[Cu(H₂O)₆]²⁺ (d⁹): t_2g⁶e_g³ — one hole. By hole formalism it is like d¹ inverted: the single transition ²E_g → ²T_2g appears as one broad, asymmetric band around 12,500 cm⁻¹ (≈ 800 nm) in the red region, so the solution looks blue. The band is broad because the ground state is Jahn–Teller distorted (tetragonal), which splits the levels into several close transitions.

Likely exam questions

1. (Long, 8 marks) Explain the splitting of d-orbitals in octahedral and tetrahedral complexes according to crystal field theory. Calculate the CFSE of [Fe(H2O)6]2+ and [Fe(CN)6]4−.
2. (Long, 8 marks) State the Jahn–Teller theorem and the Laporte and spin selection rules. Discuss the electronic spectra of [Ti(H2O)6]3+ and [Cu(H2O)6]2+.
3. (Short, 4 marks) Give any four limitations of valence bond theory.
4. (Short, 4 marks) What are the factors affecting the magnitude of 10Dq?
5. (Short, 4 marks) Explain the hole formalism principle.

Syllabus topics (RTMNU)

• Method of determining Magnetic Susceptibility by Gouy's Method
• Spin only formula and orbital contribution to magnetic moment
• Magnetic properties of Octahedral and Tetrahedral complexes with respect to CFT
• Numericals on magnetic moments
• Thermodynamic and Kinetic stability of metal complexes, their relation
• Stepwise stability and overall stability constant and their relationship
• Factors affecting the Stability of complexes
• Determination of composition of Fe(III)-SSA complex by Mole Ratio and Job's Method

Magnetic Susceptibility by Gouy's Method

Substances with unpaired electrons are paramagnetic (attracted into a magnetic field); those with all electrons paired are diamagnetic (weakly repelled). The extent is measured as magnetic susceptibility (χ).

Gouy's method:

1. The powdered sample is packed uniformly in a long glass tube (Gouy tube).
2. The tube hangs from one arm of a sensitive balance so that its lower end lies between the poles of an electromagnet and its upper end is in almost zero field.
3. The sample is weighed without the field (W₁) and with the field on (W₂).
4. A paramagnetic sample is pulled into the field and appears heavier; a diamagnetic sample appears lighter.
5. The change in weight (ΔW) is proportional to the susceptibility. The apparatus is calibrated with a standard such as Hg[Co(NCS)₄]. From χ (per gram) the molar susceptibility χ_M is found, corrected for diamagnetism of ligands.

Numerical: A complex has χ_M = 0.0101 cm³ mol⁻¹ at 298 K. μ_eff = 2.828 √(0.0101 × 298) = 2.828 × 1.735 = 4.91 BM → about 4 unpaired electrons.

Spin-Only Formula and Orbital Contribution

The magnetic moment arises from electron spin and orbital motion. For most 3d complexes the orbital part is largely quenched by the ligand field, so the spin-only formula works well:

Orbital contribution occurs only when an electron can move between degenerate orbitals that are related by rotation — i.e. when the t_2g set is unequally filled (T ground terms), e.g. octahedral d¹, d², high-spin d⁶, d⁷. Then the observed moment is higher than the spin-only value (e.g. high-spin Co²⁺ octahedral: 4.7–5.2 BM against 3.87). For A and E ground terms (d³, d⁸, high-spin d⁵, d⁹), the orbital contribution is quenched and spin-only values fit.

Magnetic Properties of Octahedral and Tetrahedral Complexes (CFT)

In octahedral complexes of d⁴–d⁷ ions, the number of unpaired electrons depends on the ligand field: weak-field ligands (F⁻, H₂O) give high-spin complexes (more unpaired electrons) and strong-field ligands (CN⁻, NH₃ for Co³⁺) give low-spin complexes. Tetrahedral complexes have small Δ_t and are always high-spin.

Numerical 1: A Cr complex shows μ = 3.9 BM. √[n(n+2)] = 3.9 → n(n+2) ≈ 15.2 → n = 3. So Cr is Cr³⁺ (d³).

Numerical 2: Calculate μ_s for [Mn(H₂O)₆]²⁺. Mn²⁺ = d⁵, weak field, n = 5 → μ = √(5 × 7) = √35 = 5.92 BM.

Thermodynamic and Kinetic Stability of Complexes and Their Relation

Thermodynamic stability tells how far a complex forms at equilibrium — measured by the stability (formation) constant; it relates to ΔG° = −2.303RT log β. Kinetic stability tells how fast a complex exchanges its ligands. Complexes that exchange ligands quickly are labile; those that react slowly are inert (Taube: a labile complex reacts completely within about 1 minute at 25 °C at 0.1 M).

Relation: the two are not necessarily linked. [Ni(CN)₄]²⁻ is thermodynamically very stable (huge β) yet exchanges CN⁻ rapidly (labile). [Co(NH₃)₆]³⁺ is thermodynamically unstable in acid (should decompose to Co³⁺ and NH₄⁺) but survives for days because it is kinetically inert. In general, d³ and low-spin d⁶ octahedral complexes (Cr³⁺, Co³⁺) are inert.

Stepwise and Overall Stability Constants and Their Relationship

Complex formation in solution takes place step by step:

Usually K₁ > K₂ > K₃… because fewer positions remain for incoming ligands (statistical factor), the charge on the complex decreases, and steric crowding increases. Numerical: for [Cu(NH₃)₄]²⁺, if log K₁ = 4.0, log K₂ = 3.3, log K₃ = 2.7 and log K₄ = 2.0, then log β₄ = 12.0, β₄ = 10¹² (illustrative values).

Factors Affecting the Stability of Complexes

(A) Metal ion:

1. Charge and size: higher charge and smaller radius (high charge density) give more stable complexes, e.g. Fe³⁺ > Fe²⁺.
2. Irving–Williams order for M²⁺ ions: Mn²⁺ < Fe²⁺ < Co²⁺ < Ni²⁺ < Cu²⁺ > Zn²⁺ (explained by size and CFSE).
3. Hard/soft nature (HSAB): hard metals (Al³⁺, Fe³⁺) prefer hard ligands (F⁻, O-donors); soft metals (Ag⁺, Hg²⁺, Pt²⁺) prefer soft ligands (I⁻, S-donors, CN⁻).

(B) Ligand:

1. Basicity: more basic ligands donate electrons better and form more stable complexes.
2. Chelate effect: multidentate ligands form far more stable complexes than similar monodentate ones (e.g. [Ni(en)₃]²⁺ > [Ni(NH₃)₆]²⁺) mainly due to a favourable entropy change. Five- and six-membered rings are most stable; EDTA forms very stable 1:1 complexes.
3. Macrocyclic effect: cyclic multidentate ligands (porphyrin, crown ethers) give even more stability.
4. Steric hindrance: bulky groups near the donor atom reduce stability.
5. π-bonding ability of ligands (CN⁻, CO) increases stability with low-valent metals.

Composition of Fe(III)–SSA Complex: Mole Ratio and Job's Methods

Fe³⁺ forms coloured complexes with 5-sulphosalicylic acid (SSA). At pH about 2–3 a violet 1:1 complex forms (λ_max about 500 nm); at higher pH red (1:2) and yellow (1:3) complexes form, so pH is kept constant with a buffer. Composition is found by spectrophotometry (colorimetry).

Job's method of continuous variation: equimolar solutions of Fe³⁺ and SSA are mixed in different proportions (e.g. 1:9, 2:8 … 9:1) keeping the total volume and total moles constant. The absorbance of each mixture is measured at λ_max and plotted against the mole fraction of ligand. The plot rises to a maximum and falls; the maximum at mole fraction x gives the ratio. For Fe(III)–SSA the maximum is at x = 0.5 → 1:1 complex (x = 0.67 would mean 1:2).

Mole ratio method (Yoe and Jones): the concentration of Fe³⁺ is kept constant and the amount of SSA is increased step by step. Absorbance is plotted against the mole ratio [L]/[M]. Absorbance rises linearly and then levels off when all the metal has formed the complex; the two straight lines meet (break point) at L : M = 1, showing a 1:1 complex.

Likely exam questions

1. (Long, 8 marks) Describe Gouy's method for determining magnetic susceptibility. Explain the spin-only formula and calculate the magnetic moments of [FeF6]3− and [Fe(CN)6]3−.
2. (Long, 8 marks) Define stepwise and overall stability constants and derive the relation between them. Discuss the factors affecting the stability of complexes.
3. (Short, 4 marks) Distinguish between thermodynamic and kinetic stability with examples.
4. (Short, 4 marks) Explain Job's method of continuous variation for the Fe(III)–SSA complex.
5. (Short, 4 marks) A complex has a magnetic moment of 2.83 BM. How many unpaired electrons does it have?

Syllabus topics (RTMNU)

• Organometallic Chemistry: Definition, Nomenclature and Classification of Organometallic compounds
• Preparation, properties and application of Alkyl and Aryls of Li
• A brief account of metal ethylenic complexes (Structure only)
• Homogeneous Hydrogenation (Wilkinson's Catalyst reaction)
• Metal carbonyls: Definition, classification, preparation, properties
• Structure and bonding in mononuclear carbonyls - Ni(CO)4, Fe(CO)5 and Cr(CO)6 with respect to back pi-bonding

Organometallic Compounds: Definition and Nomenclature

Organometallic compounds are compounds containing at least one direct bond between a metal (or metalloid such as B, Si, As) and a carbon atom of an organic group. Examples: CH₃Li, C₂H₅MgBr (Grignard reagent), (C₂H₅)₄Pb, ferrocene, Ni(CO)₄. Compounds like metal cyanides and carbides are usually not included; metal carbonyls are included. The first organometallic compound was Zeise's salt (1827); Frankland prepared diethylzinc (1849).

Nomenclature: name the organic groups first (alphabetically) and then the metal as one word — (CH₃)₂Zn dimethylzinc, C₆H₅Li phenyllithium, CH₃MgI methylmagnesium iodide, (C₂H₅)₄Pb tetraethyllead. For π-complexes the hapticity η (number of carbon atoms bonded to the metal) is given: ferrocene = bis(η⁵-cyclopentadienyl)iron(II); Zeise's salt = potassium trichlorido(η²-ethene)platinate(II); (C₆H₆)₂Cr = bis(η⁶-benzene)chromium(0).

Classification of Organometallic Compounds

1. Ionic organometallics: formed by highly electropositive metals (Na, K, Ca), e.g. C₅H₅⁻Na⁺, (C₆H₅)₃C⁻Na⁺; very reactive, insoluble in hydrocarbons.
2. σ-bonded (covalent) organometallics: normal two-electron M–C σ bonds, formed by elements of moderate electronegativity — (CH₃)₂Zn, (CH₃)₂Hg, (CH₃)₄Si, R–MgX.
3. Electron-deficient (multicentre / bridged) organometallics: Li, Be, Al compounds where bridging alkyls form 3-centre 2-electron bonds — (CH₃Li)₄, Al₂(CH₃)₆.
4. π-bonded organometallics: transition metals bonded to unsaturated organic molecules through their π-electrons — Zeise's salt, ferrocene, dibenzenechromium. Metal carbonyls are a related class.

They may also be classified by hapticity (η¹ to η⁸) or by the number of electrons donated by the ligand.

Alkyls and Aryls of Lithium: Preparation

1. From alkyl/aryl halides and lithium metal (in dry ether or hexane, under N₂/Ar): R–X + 2 Li → R–Li + LiX; e.g. n-C₄H₉Cl + 2 Li → n-C₄H₉Li + LiCl; C₆H₅Br + 2 Li → C₆H₅Li + LiBr.
2. Metal–halogen exchange: n-BuLi + Ar–Br → Ar–Li + n-BuBr (fast at low temperature).
3. Transmetallation (from organomercury): R₂Hg + 2 Li → 2 R–Li + Hg.
4. Metallation (deprotonation) of acidic C–H by n-BuLi: n-BuLi + C₆H₅C≡CH → C₆H₅C≡CLi + C₄H₁₀.

Properties and Applications of Organolithium Compounds

Properties: colourless liquids or solids, soluble in hydrocarbons and ethers; exist as aggregates (methyllithium is a tetramer, n-butyllithium a hexamer in hexane) with multicentre bonds; very reactive — react violently with water and air (t-BuLi is pyrophoric), so handled under inert gas. The C–Li bond is highly polar (C^δ−–Li^δ+), so RLi acts as a very strong base and nucleophile (more reactive than Grignard reagents).

Applications: (1) n-BuLi is an initiator for anionic polymerisation (synthetic rubbers, styrene–butadiene); (2) strong base for preparing LDA and other anions; (3) synthesis of alcohols, acids and ketones; (4) preparation of other organometallics (B, Si, Sn, Cu — Gilman reagent R₂CuLi) by transmetallation.

Metal Ethylenic Complexes (Structure Only)

The best-known example is Zeise's salt, K[PtCl₃(η²-C₂H₄)]·H₂O. Platinum(II) is square planar: three Cl⁻ and the midpoint of the ethylene C=C occupy the four corners. The C=C axis is perpendicular to the PtCl₃ plane, and the C=C bond is slightly longer (about 1.37 Å) than in free ethylene (1.34 Å); the H atoms bend back away from Pt.

Bonding (Dewar–Chatt–Duncanson model): (i) σ-donation of the filled π-electrons of ethylene into an empty metal orbital; (ii) π back-donation from a filled metal d-orbital into the empty π* orbital of ethylene. Back-donation weakens and lengthens the C=C bond.

Homogeneous Hydrogenation: Wilkinson's Catalyst

Wilkinson's catalyst is [RhCl(PPh₃)₃], chloridotris(triphenylphosphine)rhodium(I) — a square-planar, 16-electron Rh(I) complex, soluble in organic solvents. It hydrogenates alkenes and alkynes with H₂ at room temperature and 1 atm in benzene — homogeneous catalysis (catalyst and reactants in the same phase).

Catalytic cycle:

1. Ligand dissociation: RhCl(PPh₃)₃ loses one PPh₃ (or a solvent replaces it) → RhCl(PPh₃)₂.
2. Oxidative addition of H₂ → RhH₂Cl(PPh₃)₂ (Rh(III)).
3. Alkene coordination to the vacant site.
4. Migratory insertion: one hydride moves to the alkene → Rh–alkyl hydride.
5. Reductive elimination of the alkane (R–H), regenerating the Rh(I) catalyst.

Advantages: mild conditions; selective — reduces less-hindered C=C bonds and does not reduce C=O, COOR, NO₂ or CN groups; no isomerisation of most alkenes; useful in drug synthesis.

Metal Carbonyls: Definition, Classification, Preparation and Properties

Metal carbonyls are complexes of metals (usually in zero or low oxidation state) with carbon monoxide as ligand, e.g. Ni(CO)₄, Fe(CO)₅, Cr(CO)₆.

Classification: (1) Mononuclear — one metal atom: Ni(CO)₄, Fe(CO)₅, Cr(CO)₆, Mo(CO)₆; (2) Polynuclear — two or more metal atoms, with M–M bonds and often bridging CO: Mn₂(CO)₁₀, Co₂(CO)₈, Fe₂(CO)₉, Fe₃(CO)₁₂; they may be homonuclear or heteronuclear. CO may be terminal or bridging.

Preparation:

1. Direct synthesis: Ni + 4 CO → Ni(CO)₄ (room temperature, 1 atm); Fe + 5 CO → Fe(CO)₅ (about 200 °C, high pressure).
2. Reductive carbonylation: CrCl₃ + Al + 6 CO → Cr(CO)₆ + AlCl₃ (in benzene, with AlCl₃, under pressure).
3. Photolysis / heating: 2 Fe(CO)₅ → (hν) Fe₂(CO)₉ + CO.

Properties: Ni(CO)₄ — colourless, volatile, highly toxic liquid (b.p. 43 °C); Fe(CO)₅ — yellow liquid (b.p. 103 °C); Cr(CO)₆ — white crystalline solid. They are covalent, soluble in organic solvents and usually diamagnetic; most obey the 18-electron rule. They undergo substitution (Ni(CO)₄ + PPh₃ → Ni(CO)₃(PPh₃) + CO), react with halogens and decompose on heating — Ni(CO)₄ → Ni + 4 CO at about 180–200 °C (Mond process for pure nickel).

Structure and Bonding in Ni(CO)₄, Fe(CO)₅ and Cr(CO)₆: Back π-Bonding

Synergic (σ + π) bonding: (1) σ-bond: the lone pair on the carbon atom of CO is donated into an empty hybrid orbital of the metal (M ← CO). (2) π back-bond: filled metal d-orbitals (d_π) donate electron density back into the empty π* antibonding orbitals of CO (M → CO). Each effect strengthens the other — σ-donation builds up negative charge on the low-valent metal, which back-donation removes. Hence the name synergic bonding.

Evidence: (i) the M–C bond is shorter than a single bond (partial double-bond character); (ii) the C–O bond becomes longer and weaker because electrons enter π*; IR stretching frequency ν(CO) falls from 2143 cm⁻¹ in free CO to about 1850–2120 cm⁻¹ for terminal CO and about 1700–1850 cm⁻¹ for bridging CO; (iii) metals in zero oxidation state form stable compounds only because of back-bonding.

Likely exam questions

1. (Long, 8 marks) Define organometallic compounds and classify them with examples. Give the preparation, properties and applications of alkyl and aryl lithium compounds.
2. (Long, 8 marks) What are metal carbonyls? Describe the structure and bonding in Ni(CO)4, Fe(CO)5 and Cr(CO)6 with reference to back π-bonding.
3. (Short, 4 marks) Describe the structure of Zeise's salt.
4. (Short, 4 marks) Explain homogeneous hydrogenation using Wilkinson's catalyst.
5. (Short, 4 marks) Give two methods of preparation of metal carbonyls.

Syllabus topics (RTMNU)

• Metallurgy: Definition, occurrence of metals
• Concept of minerals and Ores
• Physical methods of Concentration of ores: Gravity separation, magnetic separation, froth floatation process
• Chemical methods of concentration: Leaching, Bayer's process, cyanide process
• Basic divisions: Pyrometallurgy, hydrometallurgy and electrometallurgy
• Basic metallurgical operations: Pulverisation, Calcination, Roasting, Refining, Smelting
• Furnaces in metallurgy - Blast furnace, Reverberatory furnace, Bessemer convertor
• Extraction of Copper from its ore
• Extraction of Aluminium from its ore

Metallurgy: Definition, Occurrence, Minerals and Ores

Metallurgy is the science and technology of extracting metals from their ores and refining them for use.

Occurrence: very unreactive metals (Au, Pt, some Ag and Cu) occur native (free state). Most metals occur combined as oxides, sulphides, carbonates, halides, silicates and sulphates in the earth's crust. Aluminium is the most abundant metal in the crust, followed by iron.

Mineral: a naturally occurring chemical substance in the earth's crust from which a metal may or may not be extracted. Ore: a mineral from which a metal can be extracted economically and conveniently. So all ores are minerals, but all minerals are not ores (e.g. clay is a mineral of Al, but bauxite is its ore). The unwanted earthy impurities in an ore are called gangue (matrix).

Physical Methods of Concentration of Ores

Concentration (ore dressing / benefaction) means removal of gangue from the ore.

1. Gravity separation (hydraulic washing / levigation): based on difference in density. The powdered ore is washed with a stream of water on a sloping, vibrating table (Wilfley table); light gangue is washed away and heavy ore particles settle. Used for oxide ores like haematite and cassiterite, and for native gold.
2. Magnetic separation: based on magnetic properties. Powdered ore is dropped on a conveyor belt moving over a magnetic roller; magnetic particles fall close to the roller, non-magnetic ones farther away. Used to separate magnetite (Fe₃O₄), chromite, and wolframite (magnetic) from cassiterite (non-magnetic).
3. Froth flotation process: used for sulphide ores. Finely powdered ore is mixed with water, a collector (pine oil, xanthates — make ore particles water-repellent) and a froth stabiliser (cresols, aniline). Air is blown through; the ore particles stick to the oil-coated air bubbles and rise in the froth, which is skimmed off; gangue, wetted by water, sinks. Depressants (e.g. NaCN) allow selective separation — NaCN prevents ZnS from frothing so that PbS comes into the froth.

Chemical Methods of Concentration: Leaching, Bayer's Process, Cyanide Process

Leaching is treating the powdered ore with a suitable reagent (acid, base, or other solution) that dissolves the ore (metal compound) selectively, leaving the gangue undissolved.

Bayer's process (leaching of bauxite):

Cyanide process (MacArthur–Forrest) for gold and silver: the crushed ore is leached with dilute NaCN (or KCN) solution in the presence of air; the metal dissolves as a soluble cyano complex, which is then reduced by zinc dust.

Basic Divisions: Pyrometallurgy, Hydrometallurgy and Electrometallurgy

1. Pyrometallurgy: extraction at high temperature — roasting, smelting, reduction with carbon in furnaces. Used for Fe, Cu, Zn, Pb, Sn.
2. Hydrometallurgy: extraction using aqueous solutions — leaching followed by precipitation with a more reactive metal. Used for Au, Ag (cyanide process) and low-grade Cu ores (Cu²⁺ + Fe → Cu + Fe²⁺).
3. Electrometallurgy: extraction or refining by electrolysis of fused salts or solutions. Used for highly reactive metals Na, K, Mg, Ca, Al and for refining Cu.

Basic Metallurgical Operations

1. Pulverisation: crushing large lumps in jaw/gyratory crushers and grinding to fine powder in ball mills or stamp mills, so that concentration works well.
2. Calcination: heating the ore below its melting point in the absence or limited supply of air to remove moisture, water of crystallisation, CO₂ and volatile matter, making it porous. CaCO₃ → CaO + CO₂; ZnCO₃ → ZnO + CO₂; Al₂O₃·2H₂O → Al₂O₃ + 2 H₂O.
3. Roasting: heating the ore below its melting point in excess air; used for sulphide ores; converts sulphides to oxides and removes S, As, Sb as volatile oxides. 2 ZnS + 3 O₂ → 2 ZnO + 2 SO₂; 2 PbS + 3 O₂ → 2 PbO + 2 SO₂.
4. Smelting: heating the roasted/calcined ore with a reducing agent (coke) and a flux above the melting point of the metal; the metal is obtained in molten state and impurities combine with flux to form fusible slag. ZnO + C → Zn + CO; Fe₂O₃ + 3 CO → 2 Fe + 3 CO₂; CaO (flux) + SiO₂ (gangue) → CaSiO₃ (slag). Acidic gangue needs a basic flux (CaO) and basic gangue (FeO) needs an acidic flux (SiO₂).
5. Refining: purification of the crude metal — distillation (Zn, Hg), liquation (Sn, Pb), poling (Cu), electrolytic refining (Cu, Ag, Al), zone refining (Si, Ge), and vapour-phase methods: Mond process (Ni) and van Arkel process (Ti, Zr).

Furnaces in Metallurgy

1. Blast furnace: a tall (about 25–30 m) vertical shaft of steel lined with fire bricks. Charge (ore + coke + limestone) is fed from the top through a cup-and-cone arrangement; a blast of hot air enters near the bottom through tuyeres. Zones: combustion zone at the bottom (C + O₂ → CO₂, about 2000 K), fusion/slag zone, and reduction zone at the top (Fe₂O₃ + 3 CO → 2 Fe + 3 CO₂, about 700–900 K). Molten iron and slag collect at the hearth and are tapped separately. Used for smelting iron (pig iron), also Cu and Pb ores.
2. Reverberatory furnace: the charge lies on a shallow hearth and is heated by hot flames and gases from a separate fire-box, reflected (reverberated) from the concave roof; the charge does not touch the fuel. Can be used with oxidising or reducing atmosphere. Used for roasting, smelting of copper pyrites to matte, and for making wrought iron.
3. Bessemer converter: a pear-shaped steel vessel lined with silica (acidic lining) or MgO/dolomite (basic lining), mounted on trunnions so it can be tilted. Molten matte or pig iron is poured in and a blast of air is blown through tuyeres at the bottom; impurities (Fe, S, Si, Mn, C) are oxidised and removed as gases or slag. Used to convert copper matte to blister copper and pig iron to steel.

Extraction of Copper from its Ore (Copper Pyrites, CuFeS₂)

1. Concentration: crushed ore is concentrated by froth flotation.
2. Roasting in a reverberatory furnace in a current of air: removes As and Sb as volatile oxides and partly oxidises the sulphides: 2 CuFeS₂ + O₂ → Cu₂S + 2 FeS + SO₂; 2 FeS + 3 O₂ → 2 FeO + 2 SO₂.
3. Smelting with silica (flux) in a blast or reverberatory furnace: FeO + SiO₂ → FeSiO₃ (slag). The molten mixture of Cu₂S and some FeS is called matte.
4. Bessemerisation in a Bessemer converter: air is blown through molten matte with silica. Remaining FeS → FeO → FeSiO₃ slag. Then 2 Cu₂S + 3 O₂ → 2 Cu₂O + 2 SO₂, and self-reduction: 2 Cu₂O + Cu₂S → 6 Cu + SO₂. The escaping SO₂ gives the solidified metal a blistered surface — blister copper (about 98% pure).
5. Refining: (a) poling — stirring molten copper with green wooden poles reduces Cu₂O; (b) electrolytic refining — anode: impure copper; cathode: thin sheet of pure copper; electrolyte: CuSO₄ + dilute H₂SO₄. Cu dissolves at the anode (Cu → Cu²⁺ + 2e⁻) and deposits at the cathode (Cu²⁺ + 2e⁻ → Cu), giving 99.9%+ pure copper. Less reactive metals (Au, Ag, Pt) collect below the anode as anode mud.

Extraction of Aluminium from its Ore (Bauxite)

1. Concentration of bauxite to pure alumina by Bayer's process (above). (Hall's and Serpeck's processes are other methods.)
2. Electrolytic reduction (Hall–Héroult process): pure alumina is dissolved in molten cryolite (Na₃AlF₆) with a little CaF₂. Cryolite lowers the melting point from about 2050 °C to about 950 °C and increases conductivity. Cell: an iron tank lined with carbon acts as the cathode; graphite rods dipping into the melt act as the anode.
3. Electrode reactions: cathode: Al³⁺ + 3 e⁻ → Al (molten Al collects at the bottom and is tapped); anode: 2 O²⁻ → O₂ + 4 e⁻; the oxygen burns the carbon anodes (C + O₂ → CO₂; 2C + O₂ → 2CO), so anodes must be replaced from time to time. A layer of powdered coke on top reduces heat loss.
4. Refining: Hoopes process (three-layer electrolytic cell) gives about 99.98% pure aluminium.

Likely exam questions

1. (Long, 8 marks) Describe the extraction of copper from copper pyrites.
2. (Long, 8 marks) Describe the froth flotation process and Bayer's process. Explain the extraction of aluminium from alumina by electrolysis.
3. (Short, 4 marks) Distinguish between calcination and roasting.
4. (Short, 4 marks) Distinguish between a mineral and an ore.
5. (Short, 4 marks) Describe the Bessemer converter.