Part 1: Electronic structure, ligands and complex shapes

Connect electron configurations to the transition-element definition, then count coordinate bonds to determine complex charge and geometry.

d-block and transition element are not interchangeable

A transition element forms at least one ion with an incomplete d subshell. In Period 4, Ti–Cu meet the OCR definition. Sc normally forms Sc³⁺ with 3d⁰, and Zn forms Zn²⁺ with 3d¹⁰, so neither qualifies despite both being d-block elements. Cu qualifies because Cu²⁺ is 3d⁹ even though Cu⁺ is 3d¹⁰.

Build the atom’s configuration from its atomic number, then remove 4s electrons before 3d electrons when forming these ions. Cr and Cu have the familiar atomic exceptions [Ar]3d⁵4s¹ and [Ar]3d¹⁰4s¹. They are not obtained by mechanically filling 4s² in every atom.

For Fe, [Ar]3d⁶4s² becomes Fe²⁺ [Ar]3d⁶ and Fe³⁺ [Ar]3d⁵. Bracket notation is useful for learning; expand the core to 1s²2s²2p⁶3s²3p⁶ when the question asks for a full configuration. Always check the electron total against atomic number minus charge.

Recognise characteristic behaviour without making it universal

Transition elements commonly show variable oxidation states, coloured compounds and catalytic activity. Iron provides Fe²⁺ and Fe³⁺, while copper provides Cu⁺ and Cu²⁺. Aqueous Cu²⁺ is usually blue and Fe²⁺ pale green; Fe³⁺ solutions may appear yellow/brown depending on conditions.

Not every ion of every transition element is coloured. Oxidation state and ligand environment matter. OCR does not require detailed d-orbital splitting to explain colour here; use observed colours accurately without adding an unsupported universal colour rule.

Examples of catalytic behaviour include iron in ammonia manufacture, nickel in hydrogenation and MnO₂ in hydrogen peroxide decomposition. A catalyst supplies an alternative pathway and is regenerated overall. Reduced energy demand can be beneficial, but toxicity, supply and recovery of the catalyst still matter in an industrial evaluation.

A ligand supplies the electron pair

A ligand is an ion or molecule that donates an electron pair to a central metal atom or ion, forming a coordinate bond. A complex consists of the metal and surrounding ligands; it may be a charged complex ion or neutral. The arrow convention for a coordinate bond points from the donor towards the acceptor.

H₂O, NH₃ and Cl⁻ are monodentate: each uses one donor atom to form one coordinate bond. Ethane-1,2-diamine, H₂NCH₂CH₂NH₂, is bidentate: one nitrogen lone pair at each end makes two bonds to the same metal. Three such ligands can therefore give coordination number six.

Coordination number counts donor bonds, not ligand molecules and not charge. In [Co(en)₃]³⁺, three neutral en ligands donate six pairs in total, so the metal oxidation state is +3 and coordination number is six. Neutral ligands do not cancel a metal’s charge.

Work from coordination number to the appropriate geometry

Six-coordinate complexes are commonly octahedral, with adjacent ligand–metal–ligand angles of 90° and opposite angles 180°. Four-coordinate complexes may be tetrahedral, around 109.5°, or square planar, with adjacent 90° angles. Coordination number four alone does not decide between them; use the metal, ligands or supplied structure.

[Cu(H₂O)₆]²⁺ is treated as octahedral in this course, while [CuCl₄]²⁻ is treated as tetrahedral. Pt(NH₃)₂Cl₂ is square planar. Real distortions need not be introduced unless relevant data are supplied.

Worked charge check: in [CuCl₄]²⁻, four chloride ligands contribute −4. The metal must be +2 for a total of −2. In [Cr(NH₃)₆]³⁺ the six neutral ammonia ligands contribute zero, leaving chromium at +3. Shape changes can occur without redox: the oxidation number must be checked independently.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources