Transition Metals and Coordination Compounds

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An element that forms at least one stable ion with a partially filled d subshell. Note: Zinc (Zn) and Scandium (Sc) are d-block elements but not transition metals by this strict definition (Zn²⁺ has full d¹⁰; Sc³⁺ has empty d⁰).

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1. Variable oxidation states. 2. Form coloured ions. 3. Act as catalysts. 4. Form complex ions with ligands. Also: high melting/boiling points, good conductors, high density.

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Their 3d and 4s electrons have similar energies — different numbers can be involved in bonding. Example: Iron: Fe²⁺ (3d⁶4s⁰) and Fe³⁺ (3d⁵4s⁰). Manganese shows the most: +2 to +7.

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They absorb specific wavelengths of visible light to promote electrons between d orbitals of different energy levels (d-d transitions) — when the d subshell is partially filled. The transmitted/reflected colour is complementary to the absorbed colour.

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Cu⁺ has the electron configuration [Ar]3d¹⁰ — a full d subshell. No d-d transitions are possible → no visible light absorbed → colourless. (Cu²⁺ with 3d⁹ is blue/green because d-d transitions are possible.)

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A central metal ion surrounded by ligands (molecules or ions that donate lone pairs). Written in square brackets: [Fe(H₂O)₆]³⁺. The charge = metal oxidation state + sum of ligand charges.

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An ion or molecule that donates a lone pair of electrons to a central metal ion, forming a coordinate (dative covalent) bond. Examples: H₂O, NH₃, Cl⁻, CN⁻, OH⁻, ethylenediamine (en).

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A ligand that forms one coordinate bond to the central metal ion. Examples: H₂O, NH₃, Cl⁻, CN⁻, CO.

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A ligand that forms two coordinate bonds to the central metal ion simultaneously. Examples: ethylenediamine (en, H₂N-CH₂-CH₂-NH₂), oxalate (C₂O₄²⁻), 2,2-bipyridine.

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A ligand that forms three or more coordinate bonds. Most famous example: EDTA⁴⁻ (ethylenediaminetetraacetate) — a hexadentate ligand that wraps around a metal ion at 6 points. Used in chelation therapy and as a preservative.

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Polydentate ligands form more stable complexes than equivalent monodentate ligands. Entropy-driven: replacing several monodentate ligands with one polydentate ligand increases the number of particles → increases entropy (ΔS > 0) → more negative ΔG.

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The number of coordinate bonds formed between the central metal ion and its ligands. Most common: 6 (octahedral) and 4 (tetrahedral or square planar). Example: [Fe(H₂O)₆]³⁺ has coordination number 6.

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Octahedral (6 ligands): Most common, e.g., [Fe(H₂O)₆]³⁺. Tetrahedral (4 ligands): e.g., [CoCl₄]²⁻. Square planar (4 ligands): Common with Pt²⁺, Pd²⁺, Au³⁺, e.g., cisplatin [Pt(NH₃)₂Cl₂].

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Order: (1) Ligands alphabetically (with prefixes di, tri, tetra for count). (2) Metal name (Latin for anion complexes: ferrate, cuprate). (3) Oxidation state in Roman numerals. Example: [Fe(H₂O)₆]³⁺ = hexaaquairon(III).

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The equilibrium constant for the formation of a complex ion. Larger Kstab = more stable complex. Example: [Fe(CN)₆]⁴⁻ has a very large Kstab → very stable → CN⁻ binds tightly to Fe²⁺.

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A protein in red blood cells that carries oxygen. Contains iron (Fe²⁺) in a haem group — a porphyrin ring complex. O₂ binds reversibly to Fe²⁺. CO binds irreversibly (~200× more strongly than O₂) → carbon monoxide poisoning.

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The green pigment in plants responsible for photosynthesis. Contains magnesium (Mg²⁺) at the centre of a porphyrin ring. Absorbs red and blue light; reflects green light (hence plants appear green).

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cis-[Pt(NH₃)₂Cl₂] — a square planar platinum complex used as a chemotherapy drug. The cis arrangement allows it to cross-link DNA strands in cancer cells, preventing replication. The trans isomer is inactive.

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The d-block spans groups 3–12 of the periodic table (periods 4–7), where d orbitals are being filled. Period 4 d-block: Sc, Ti, V, Cr, Mo, Mn, Fe, Co, Ni, Cu, Zn — 10 elements per period, 40 d-block elements total.

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Cr: [Ar]3d⁵4s¹ (not [Ar]3d⁴4s²) — half-filled d shell is extra stable. Cu: [Ar]3d¹⁰4s¹ (not [Ar]3d⁹4s²) — fully filled d shell is extra stable. These are exceptions to the Aufbau principle.

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A reaction where one ligand in a complex ion is replaced by another. Example: [Cu(H₂O)₆]²⁺ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O. Colour changes from pale blue (water complex) to deep blue (ammonia complex).

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[Cu(H₂O)₆]²⁺ (pale blue) + 2OH⁻ → [Cu(OH)₂(H₂O)₄] (pale blue/green precipitate). The precipitate is copper(II) hydroxide. It does NOT dissolve in excess NaOH. (Contrast with Al³⁺ which dissolves in excess NaOH.)

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First forms Cu(OH)₂ precipitate, then the precipitate dissolves in excess NH₃: Cu(OH)₂ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ — a deep, intense blue solution. This is the tetraamminecopper(II) complex.

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A list of ligands ranked by their ability to split d-orbital energy levels (crystal field splitting Δ). Strong-field ligands (CN⁻, CO) cause large splitting → absorb high-energy light (appear yellow/red). Weak-field ligands (Cl⁻, H₂O) cause small splitting → absorb low-energy light (appear blue/green).

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Fe: Haber Process (NH₃ synthesis). V₂O₅: Contact Process (H₂SO₄ production). Pt/Pd/Rh: Catalytic converters (car exhaust). Ni: Hydrogenation of vegetable oils (margarine). MnO₂: Decomposition of H₂O₂. Their variable oxidation states allow them to form intermediates and regenerate.