I85PREP
⌘ K
GTLE Subject Specialism Chemistry study materials
GTLE Subject Specialism Chemistry study resources

GTLE Subject Specialism Chemistry: Inorganic Chemistry: Periodicity, Coordination, and Descriptive Chemistry

GTLE Subject Specialism Chemistry · Work through your exam, one topic at a time.

All guide sections

Inorganic Chemistry: Periodicity, Coordination, and Descriptive Chemistry

Inorganic questions combine periodic trends, coordination chemistry and descriptive facts about the main groups. Expect to assign geometry and isomerism from a formula, apply crystal field theory to spin states and colour, and recall the solubility and acid-base patterns that distinguish the groups.

Coordination chemistry

  • Coordination number 4 gives tetrahedral geometry in most cases, or square planar for d8 ions such as [PtCl4]2-. Coordination number 6 gives octahedral geometry with cis/trans and fac/mer isomers; bidentate ligands such as en or oxalate can produce optical isomers.
  • Isomer types: ionisation, hydration, linkage (NO2- versus ONO-), coordination, geometric and optical.
  • Crystal field theory: Delta_o is the octahedral gap. Strong-field ligands (CN-, CO, phen, NO2-) give low spin; weak-field ligands (I-, Br-, Cl-, F-, H2O) give high spin, with high spin favoured when Delta_o is smaller than the pairing energy P.
  • Colour comes from d-d transitions; [Cu(H2O)6]2+ is pale blue because the d9 ion absorbs in the visible region.
  • The chelate effect: polydentate ligands increase entropy on binding, so [Cu(en)2]2+ is more stable than the equivalent ammine complex.

Periodicity and descriptive patterns

The inert pair effect makes Tl, Pb and Bi prefer an oxidation state two below the group maximum. Diagonal relationships exist for Li/Mg and Be/Al. In group 2, hydroxide solubility increases down the group while sulfate solubility decreases, making BaSO4 the classic precipitate. Al(OH)3 and Zn(OH)2 are amphoteric and dissolve in both acid and excess alkali. Halogen oxidising power falls down the group, and HClO4 is the strongest simple oxoacid.

Worked example

Compare [Fe(CN)6]3- and [Fe(H2O)6]3+. Both contain Fe3+, a d5 ion. Cyanide is strong field, so all five electrons occupy t2g, giving one unpaired electron and CFSE = -2.0 Delta_o + 2P. Water is weak field, so all five d orbitals are singly occupied, giving five unpaired electrons and CFSE = 0. The cyanide complex is deeply coloured and nearly diamagnetic; the aqua complex is high spin and much paler.

Traps

  • Counting d electrons from the neutral metal rather than from the ion.
  • Assigning a tetrahedral complex as low spin; Delta_t is small.
  • Confusing cis/trans with fac/mer for octahedral MA3B3.
  • Assuming all group 2 sulfates are insoluble; MgSO4 dissolves freely.
  • Forgetting that optical isomerism in octahedral complexes requires a chelating ligand set.