The d- and f-Block Elements and Coordination Compounds
This domain tests the definition and characteristic properties of transition metals, lanthanoid contraction, and the nomenclature, isomerism, bonding and magnetic behaviour of coordination compounds.
Transition elements
A transition element has an incompletely filled d subshell in its atom or in one of its common oxidation states; Zn, Cd and Hg (d¹⁰) are therefore excluded. Key properties: variable oxidation states (Mn from +2 to +7), colour from d–d transitions (Sc³⁺ d⁰ and Cu⁺ d¹⁰ are colourless), paramagnetism, catalysis, interstitial compounds, alloy formation and a strong tendency to form complexes.
Magnetism and lanthanoids
μ = √(n(n + 2)) BM, where n is the number of unpaired electrons. Fe³⁺ (d⁵) high spin: n = 5, μ = 5.92 BM; low spin: n = 1, μ = 1.73 BM. Lanthanoid contraction - the steady radius decrease from La to Lu caused by poor shielding by 4f electrons - makes Zr and Hf almost identical in size and chemistry.
Coordination compounds
- Ligands: monodentate (NH₃, Cl⁻), bidentate (en, C₂O₄²⁻), polydentate (EDTA⁴⁻); chelates are extra stable.
- Nomenclature: ligands alphabetically, then the metal with its oxidation state in Roman numerals in brackets.
- Isomerism: ionisation, hydrate, linkage, coordination, geometrical and optical.
- VBT: [Fe(CN)₆]³⁻ is inner-orbital d²sp³, low spin, one unpaired electron; [FeF₆]³⁻ is outer-orbital sp³d², high spin, five unpaired electrons.
- CFT: strong-field ligands (CN⁻, CO) give large Δo and low spin; weak-field ligands (F⁻, H₂O) give small Δo and high spin.
Worked example
Predict the magnetism of [Co(NH₃)₆]³⁺. Cobalt is +3, so d⁶. NH₃ with Co(III) gives low spin, t2g⁶ eg⁰, hence n = 0, μ = 0: the complex is diamagnetic and inner-orbital (d²sp³).
Traps that cost marks
- Counting d electrons from the neutral atom instead of the ion.
- Calling Cu⁺ coloured, although d¹⁰ gives no d–d transition.
- Writing the oxidation state as a subscript rather than Roman numerals in brackets.
- Assuming every d-block ion is paramagnetic.