The definition of a transition metal in terms of an incomplete d sub-shell. The characteristic properties of transition metals: complex formation, coloured ions, variable oxidation states and catalytic activity. The shapes of complex ions and the meaning of coordination number and ligand. Stereoisomerism in complexes. Ligand substitution reactions and the chelate effect. The origin of colour in transition metal ions and its use in colorimetry. The role of transition metals as homogeneous and heterogeneous catalysts.

What this dot point is asking

AQA wants you to define a transition metal by its d sub-shell, explain the four characteristic properties (complex formation, coloured ions, variable oxidation states and catalysis), describe the shapes and stereoisomerism of complex ions, explain ligand substitution and the chelate effect, account for colour and its use in colorimetry, and describe transition metals as catalysts.

Definition

Complex ions, ligands and coordination number

Ligands can be monodentate (one lone pair, e.g. $H_2O$, $NH_3$, $Cl^-$), bidentate (two, e.g. ethanedioate) or multidentate (e.g. EDTA, with six).

Stereoisomerism in complexes

Octahedral complexes with two different ligands can show cis-trans isomerism (e.g. cisplatin, used as an anti-cancer drug, is the cis isomer of $[Pt(NH_3)_2Cl_2]$). Octahedral complexes with three bidentate ligands can show optical isomerism (non-superimposable mirror images).

Ligand substitution and the chelate effect

One ligand can replace another. For example adding excess ammonia to aqueous copper(II):

The origin of colour and colorimetry

When ligands bond, the 3d orbitals split into two energy levels separated by an energy gap $\Delta E$. An electron absorbs visible light of frequency matching $\Delta E$ and is promoted to the higher level; the complementary colour is transmitted, so the ion appears coloured. The relationship is:

Changing the oxidation state, ligand, or coordination number changes $\Delta E$ and so the colour. Ions with $d^0$ or $d^{10}$ (e.g. $Sc^{3+}$, $Zn^{2+}$) are colourless. Colorimetry uses the depth of colour to find an unknown concentration against a calibration curve.

Variable oxidation states and catalysis

Transition metals show variable oxidation states because the 3d and 4s energies are similar, so a variable number of electrons can be lost.

• Heterogeneous catalysts are in a different phase from the reactants and work by adsorbing reactants onto active sites on their surface (e.g. iron in the Haber process, vanadium(V) oxide in the Contact process).
• Homogeneous catalysts are in the same phase and work by forming an intermediate, cycling between oxidation states (e.g. $Fe^{2+}/Fe^{3+}$ catalysing the reaction between iodide and peroxodisulfate ions).

Try this

Q1. State the shape and coordination number of $[Cu(H_2O)_6]^{2+}$. [1 mark]

• Cue. Octahedral, coordination number 6.

Q2. Explain why $Zn^{2+}$ solutions are colourless. [2 marks]

• Cue. $Zn^{2+}$ is $3d^{10}$ (full d sub-shell), so no d-to-d electron promotion can occur, so no visible light is absorbed.

Q3. Name one heterogeneous and one homogeneous transition-metal catalyst and the reaction each catalyses. [2 marks]

• Cue. Iron in the Haber process (heterogeneous); $Fe^{2+}$ catalysing iodide and peroxodisulfate (homogeneous).