The d-block Elements

Part A: The Essentials

19.1 Occurrence and Recovery

The elements on the left of the 3d series occur in nature primarily as metal oxides or as metal cations in combination with oxoanions. To the right of Fe, Co, Ni, Cu, and Zn occur mainly as sulfides and arsenides, consistent with increasingly soft Lewis acid character.

19.2 Chemical and Physical Properties

Size and Electronegativity

The presence of d electrons in the valence shell is responsible for the colour, electronic conduction, magnetism, and rich organometallic chemistry of the d metals and their compounds.

Part B: The Detail - Overview

19.3 Group 3: Scandium, Yttrium, and Lanthanum

(a) Occurrence and Uses

Scandium is present at low levels in many lanthanoid ores. Commercial uses include alloying with aluminium in aerospace components (~2 tonnes/year production). ScI₃ is used in high-intensity mercury vapor discharge lamps producing sunlight-like output.

Yttrium (~600 tonnes/year) is used extensively in optical applications. Doped complex oxides such as Eu:YVO₄ are used as phosphors in displays and LED lighting. Yttrium aluminium garnet (YAG, Y₃Al₅O₁₂) is used in lasers. YBa₂Cu₃O₇ is used in high-temperature superconductors.

Lanthanum is used in mischmetal and as a component in nickel metal hydride rechargeable batteries. Lanthanum carbonate treats chronic kidney dysfunction by forming extremely insoluble LaPO₄ (K_sp = 3.7 × 10⁻²³).

(b) Binary Compounds

All form compounds almost exclusively in the +3 oxidation state. Oxides are white solids (M₂O₃), insoluble in water but soluble in dilute acid. A number of subhalides exist, such as Sc₇Cl₁₀ containing multiple metal–metal bonds.

(c) Complex Oxides and Halides

The ionic radius of Y³⁺ (102 pm in eight-fold coordination) is similar to many Ln³⁺ cations. Unlike lanthanoids, Y³⁺ has no unpaired f electrons, making yttrium compounds excellent host materials for doping with Ln³⁺ cations.

19.4 Group 4: Titanium, Zirconium, and Hafnium

(a) Occurrence and Uses

Titanium is the second most abundant d metal after iron. It is as strong as steel but only half as dense, corrosion-resistant and high-melting. The Kroll process involves heating TiO₂ with chlorine and carbon to give TiCl₄, then reducing with molten magnesium under argon. TiO₂ is widely used as a white pigment.

Zirconium is highly transparent to neutrons and is used as cladding material for fuel rods in nuclear power plants. Hafnium is a highly effective neutron absorber in control rods.

(b) Binary Compounds

Chemistry dominated by the +4 oxidation state. MO₂ compounds are high-melting solids. MX₄ (X = Cl, Br, I) are volatile covalent liquids/low-melting solids with tetrahedral geometries and are strong Lewis acids.

(c) Complex Oxides

In BaTiO₃, the distorted perovskite structure gives a very high dielectric constant (~2000× that of air), used in charge-storage capacitors. Lead zirconate titanate (PZT) shows a strong piezoelectric effect for transducers.

Yttria-stabilized zirconia (YSZ): Doping ZrO₂ with Y₂O₃ introduces oxide vacancies. Above 750°C, oxide ions diffuse rapidly, enabling applications in oxygen sensors and solid oxide fuel cells.

(e) Organometallic Compounds

Group 4 organometallics are normally electron-deficient (16-electron). Compounds of type [M(Cp)₂XY] are active alkene polymerization catalysts for highly stereoregular polymers.

19.5 Group 5: Vanadium, Niobium, and Tantalum

(a) Occurrence and Uses

Vanadium occurs in 60+ minerals and fossil fuels. Used to harden steel and in high-speed tools. V₂O₅ catalyzes sulfuric acid production. Vanadium is essential to many species, found in haloperoxidases and an alternative nitrogenase.

Niobium and tantalum have very similar properties. Niobium–tin (Nb₃Sn) alloys are superconducting at 4.2 K and used in NMR spectrometer and MRI scanner magnets.

(b) Binary Compounds

V₂O₅ structure consists of sheets of V(=O)O₄ square-based pyramids. Depending on pH, the oxide form varies from [MO₄]³⁻ to [MO₂]⁺, with many polyoxometallates in between.

19.6 Group 6: Chromium, Molybdenum, and Tungsten

(a) Occurrence and Uses

Chromium (40% from South Africa) is extremely hard and corrosion-resistant. Used in stainless steels (10-40%) and chrome plating. Essential to humans for glucose regulation.

Molybdenum (80% used in steels, up to 10%). Essential to all species in 20+ enzymes for oxygen atom transfer. Found in the FeMo cofactor of nitrogenase.

Tungsten has the highest melting point of any metal (filaments in incandescent lightbulbs). Tungsten carbide tips drill bits and saw blades. Only third-row d metal with a biological role (formate dehydrogenase in microbes).

(b) Binary Compounds

CrO₃ (chains of vertex-sharing tetrahedra) and [CrO₄]²⁻/[Cr₂O₇]²⁻ are strongly oxidizing. MoO₃ and WO₃ (linked MO₆ octahedra) are not rapidly oxidizing—reflecting increased stability of high oxidation states for 4d/5d metals.

CrO₂ (rutile structure) exhibits excellent ferromagnetic properties, used in high-quality audio/video tapes. Cr₂O₃ (viridian) is a widely used green pigment.

MS₂ (M = Mo, W) adopt layered structures used as solid-state lubricants.

(c) Complex Oxides: Bronzes

Tungsten bronzes (M_xWO₃, 0 < x ≤ 1) are nonstoichiometric compounds with metallic properties. While WO₃ is an insulator (d⁰), alkali metal presence reduces W and introduces electrons into a conduction band, giving high electrical conductivity and lustre.

(e) Organometallic Compounds

The 18-electron neutral hexacarbonyls [M(CO)₆] are extremely stable white solids that can be handled in air. Bis(arene) complexes and cyclopentadienyl derivatives are well known. Tungsten forms alkylidene and alkylidyne complexes with formal M=C and M≡C bonds.

19.7 Group 7: Manganese, Technetium, and Rhenium

(a) Occurrence and Uses

Manganese (80% reserves in South Africa as pyrolusite MnO₂). ~500 billion tonnes exist in ocean floor nodules. Used in steel (1-13%) and alkaline batteries (MnO₂ → Mn₂O₃). Most notable biological role: the active site for photosynthetic O₂ evolution.

Technetium was the first unstable element synthesized (1936). ^99mTc (t_½ = 6 hours, β emission) is used medically as a radioactive tracer in compounds like Cardiolyte® for heart imaging.

Rhenium was the last stable nonradioactive element discovered (1925). One of the rarest metals; used in high-temperature jet engine alloys and as a platinum co-catalyst for alkane reforming.

(b) Binary Compounds

Mn forms oxides MnO, Mn₂O₃, MnO₂, and Mn₂O₇. Tc and Re only form MO₂, MO₃, and M₂O₇. The +7 oxides dissolve in water to give [MO₄]⁻ ions—permanganate is very powerful oxidizer; pertechnate and perrhenate are less oxidizing.

Binary MX₇ halides only represented by ReF₇. MnF₃/MnF₄ equilibrium is used to purify fluorine.

(d) Coordination Complexes

Mn(II) is the most stable form in aqueous solution with high-spin d⁵ configuration (stabilized by exchange energy). Mn(II) complexes have no LFSE, show little geometry preference, and are essentially colorless (no allowed d-d transitions).

Tc and Re coordination chemistry is dominated by high-oxidation-state complexes with oxido and nitrido ligands.

19.8 Group 8: Iron, Ruthenium, and Osmium

(a) Occurrence and Uses

Iron is the most abundant transition metal in biology (>4 g per human). Roles include O₂ transport (haemoglobin), electron transfer, acid–base catalysis, gene regulation, and magnetic field sensing. Used as catalyst in the Haber process for ammonia synthesis.

Ruthenium is used in platinum/palladium alloys and RuO₂ in chip resistors (>50% consumption). [Ru(bpy)₃]²⁺ is a photosensitizer.

Osmium is the densest element (22.6 g cm⁻³), twice as dense as lead. OsO₄ is used in organic synthesis but is highly toxic.

(b) Binary Compounds

Fe₃O₄ (magnetite, spinel structure) may be permanently magnetized (lodestone)—used in navigational compasses for 800+ years. Fe(VI) exists in [FeO₄]²⁻ compounds. FeS₂ (iron pyrites, "fool's gold") contains the disulfide ion S₂²⁻.

Os slowly oxidizes in air to volatile OsO₄. Both RuO₄ and OsO₄ are yellow, volatile, and extremely toxic.

(c) Complex Oxides

Ferrites (AB₂O₄ spinels with B = Fe³⁺): Soft ferrites (Mn_1−xZn_xFe₂O₄) for transformer cores. Hard ferrites (SrFe₁₂O₁₉) as permanent magnets.

Iron-based superconductors: Ln(O,F)FeAs with T_c up to 53 K; LiFeAs (T_c = 18 K).

(d) Coordination Complexes

The balance between high- and low-spin for Fe(III) is delicate—complexes with low-field ligands (H₂O, halides) are high-spin; with high-field ligands (CN⁻, bpy) are low-spin. Spin-crossover complexes change spin state with temperature, pressure, or solvent.

(e) Organometallic Compounds

Ferrocene [(η⁵-C₅H₅)₂Fe]: Discovery in the 1950s kick-started modern organometallic chemistry. Very stable 18-electron sandwich complex. [Fe(CO)₅] is trigonal bipyramidal; forms clusters [Fe₂(CO)₉] and [Fe₃(CO)₁₂].

Grubbs catalyst (Ru carbene complex): Active in alkene metathesis (2005 Nobel Prize).

19.9 Group 9: Cobalt, Rhodium, and Iridium

(a) Occurrence and Uses

Cobalt compounds have been used for millennia for rich blue glass coloring (detected in Egyptian sculptures, 3rd century BC Persian jewelry, Pompeii ruins). Vitamin B₁₂ (cobalamin) has organometallic cobalt at its heart—essential to all animal life.

Rhodium (~20 tonnes/year): Highly reflective; used to coat optical fibers and mirrors. 80% used in catalytic converters. Also used in the Monsanto acetic acid process.

Iridium is the most corrosion-resistant metal (~3 tonnes/year). Used in X-ray telescope reflectors and the Cativa process for methanol carbonylation.

(b) Binary Compounds

Most stable oxidation state: +2 for cobalt, +3 for rhodium and iridium. High-oxidation-state compounds (RhF₆, IrF₆) are strongly oxidizing and often unstable.

(c) Complex Oxides

LiCoO₂ (layer structure): Used in rechargeable battery systems. CoAl₂O₄ (spinel): Deep royal blue pigment (tetrahedral Co(II)).

(d) Coordination Complexes

Co(II) forms more tetrahedral complexes than any other d metal (d⁷ minimizes LFSE difference between octahedral and tetrahedral). Octahedral complexes are pink/red; tetrahedral are intensely blue.

Rh and Ir are dominated by octahedral low-spin d⁶ M³⁺ complexes.

(e) Organometallic Compounds

16-electron square-planar Rh(I) and Ir(I) complexes (e.g., Vaska's complex) readily undergo oxidative addition to give 18-electron octahedral M(III) complexes. This enables homogeneous hydrogenation (Wilkinson's catalyst) and methanol carbonylation.

19.10 Group 10: Nickel, Palladium, and Platinum

(a) Occurrence and Uses

Nickel (60% in corrosion-resistant steel): The Mond process uses volatile [Ni(CO)₄] for purification. Important in microbial biology for H₂ oxidation, H₂ production, and CO₂ reduction. [Ni(CO)₄] is extremely toxic.

Palladium: Majority used in catalytic converters. Widely used in synthesis (hydrogenation, carbon–carbon bond forming—2010 Nobel Prize).

Platinum: Most ductile pure metal. Major uses in catalytic converters and jewelry. cis-[PtCl₂(NH₃)₂] (cisplatin) derivatives are widely used anticancer treatments—platinum binds to DNA preventing cell replication.

(b) Binary Compounds

PtF₆ is very strongly oxidizing—can oxidize both O₂ and xenon. Pd readily absorbs H₂ to form interstitial hydride with greater H density than solid H₂. Raney nickel (porous form) absorbs H₂ for delivery to organic substrates.

(c) Hydroxides and Oxides

Ni metal hydride batteries use Ni(OH)₂ ⇌ NiO(OH) transformation. LaNi₅H₆ contains more H₂ per unit volume than liquid H₂—potential hydrogen storage material.

NiO/YSZ composites: Anodes in solid oxide fuel cells.

(d) Coordination Complexes

Ni²⁺ shows octahedral, trigonal bipyramidal, square-based pyramidal, tetrahedral, and square-planar geometries. Pd²⁺ and Pt²⁺ are almost invariably square-planar (d⁸ configuration). Pt(IV) complexes are low-spin octahedral d⁶ and very inert.

(e) Organometallic Compounds

[Ni(CO)₄] (tetrahedral 18-electron) and Zeise's salt K[(CH₂=CH₂)PtCl₃] are historic compounds. Pd(II) 16-electron square-planar complexes are used in hydrogenation, Wacker process, and Pd-mediated C–C bond formation.

19.11 Group 11: Copper, Silver, and Gold

(a) Occurrence and Uses

Copper (15 million tonnes/year): Very high electrical and thermal conductivity. Reserves predicted to last only 15-20 years. Essential to biology (10+ Cu-dependent enzymes including cytochrome c oxidase). Also found in hemocyanin (blue O₂-transport protein in arthropods/molluscs).

Silver: Highest electrical and thermal conductivity of all metals. Ag⁺ is deadly to bacteria and viruses—used in medical dressings and antibacterial clothing.

Gold (~2000 tonnes/year): Most malleable metal (1 g can be beaten into >1 m² sheet). 75% used in jewelry. Colloidal gold nanoparticles vary from red to purple depending on size—used as pigments for centuries.

(b) Binary Compounds

Most stable state: +2 for Cu; +1 and +3 for Ag and Au. Cu(I) compounds disproportionate in solution. Cu₂O (linear Cu⁺ coordination) is used as a red pigment and in antifouling paint. AuF₅ and Ag(II) compounds are known but strongly oxidizing.

(c) Complex Chalcogenides: High-Temperature Superconductors

Over 50 different complex copper oxide superconductors discovered since 1986, including YBa₂Cu₃O_7-d (YBCO) and Bi₂Sr₂CaCu₂O₈ (BISCO). All contain sheets of linked CuO₄ square planes with Cu in average oxidation state between +2.15 and +2.35.

Copper indium gallium selenide (CIGS): Solar cell efficiencies near 20%.

(d) Coordination Complexes

Cu(II) d⁹ complexes are Jahn–Teller distorted. Ag⁺ d¹⁰ shows little geometry preference (linear, trigonal, tetrahedral all known). Au(III) d⁸ complexes are square-planar; Au(I) are linear.

19.12 Group 12: Zinc, Cadmium, and Mercury

(a) Occurrence and Uses

Zinc (4th most used metal, ~10 million tonnes/year): 50%+ used for galvanizing steel. ZnO used in vulcanization, medical applications (calamine, antibacterial), white pigment, UV-blocking. Second most abundant d metal in biology (200+ enzymes including carbonic anhydrase; Zn-finger transcription factors).

Cadmium: 90% used in rechargeable batteries. An accumulative poison—nonreversibly replaces zinc in enzymes.

Mercury: Unique liquid metal at room temperature (filled subshells + relativistic effects + lanthanoid contraction). Cinnabar (HgS, vermillion) used as red pigment since Palaeolithic times (~30,000 years ago). Highly toxic—uses being phased out.

(b) Binary Compounds

Zn and Cd chemistry is almost exclusively M²⁺ d¹⁰. Mercury also forms [Hg₂]²⁺ cation with Hg–Hg single bond. HgF₄ (d⁸) reported in low-temperature matrices would make Hg a transition metal.

ZnO polymorphs: wurtzite and zinc-blende (4:4 coordination); transforms to rock-salt at >10 GPa. HgO contains linear O–Hg–O units; heating decomposes to Hg and O₂ (Priestley first produced pure O₂ this way, 1774).

(d) Coordination Complexes

Both tetrahedral and octahedral geometries (d¹⁰—no CFSE preference). Zn²⁺ is borderline hard/soft; Cd²⁺ and Hg²⁺ are distinctly soft. [Hg₂]²⁺ complexes are linear X–Hg–Hg–X.

Exercises