The characteristic properties of transition metals, variable oxidation states, the formation and shapes of complex ions, ligand substitution, the origin of colour from d-orbital splitting, and the catalytic behaviour of transition metals.

What this dot point is asking

CCEA wants you to describe the characteristic properties of transition metals, explain variable oxidation states, describe the formation and shapes of complex ions and ligand substitution, explain the origin of colour from d-orbital splitting, and describe the catalytic behaviour of transition metals.

Characteristic properties

The variable oxidation states arise because the $4s$ and $3d$ sub-shells are close in energy, so successive electrons can be lost without a large jump in ionisation energy, letting an element such as manganese exist in states from $+2$ to $+7$. The higher oxidation states are usually found combined with oxygen or fluorine (as in $\text{MnO}_4^-$ or $\text{VO}_2^+$), and the relative stability of the states underlies the redox chemistry exploited in titrations.

Complex ions and ligand substitution

The number of dative bonds the metal forms is its coordination number: six is the most common (octahedral, as in $[\text{Fe}(\text{H}_2\text{O})_6]^{2+}$), but bulky ligands such as chloride give four (tetrahedral, as in $[\text{CuCl}_4]^{2-}$). The overall charge on the complex is the metal-ion charge plus the charges of any ionic ligands. Ligand substitution can be partial or complete and often produces a striking colour change: adding concentrated ammonia to pale blue $[\text{Cu}(\text{H}_2\text{O})_6]^{2+}$ gives the deep royal-blue $[\text{Cu}(\text{NH}_3)_4(\text{H}_2\text{O})_2]^{2+}$, and adding concentrated hydrochloric acid gives the yellow-green tetrahedral $[\text{CuCl}_4]^{2-}$. These reactions are used to test for and identify transition-metal ions.

The origin of colour

Because the colour depends on the size of $\Delta E$, changing the ligand changes the colour: a stronger-field ligand widens the gap, so higher-energy (shorter-wavelength) light is absorbed and the transmitted colour shifts. This is why $\text{Sc}^{3+}$ (empty d sub-shell) and $\text{Zn}^{2+}$ (full d sub-shell) are colourless, there being no partly filled d level for an electron to be promoted within. The intensity of the colour is the basis of colorimetry, used to find the concentration of a coloured ion from a calibration graph.

Catalysis

Transition metals and their compounds are good catalysts because their variable oxidation states let them accept and donate electrons readily, providing an alternative reaction pathway of lower activation energy. In heterogeneous catalysis the reactants adsorb onto the metal surface, react, and the products leave (iron in the Haber process, $\text{V}_2\text{O}_5$ in the Contact process, nickel in hydrogenation). In homogeneous catalysis the catalyst is in the same phase and works by changing oxidation state during the cycle, as $\text{Fe}^{2+}/\text{Fe}^{3+}$ does in catalysing the reaction between $\text{S}_2\text{O}_8^{2-}$ and $\text{I}^-$.

Examples in context

Transition-metal chemistry is everywhere in industry and biology. The intense colours of complexes give pigments and gemstones their hue (the green of emerald and the red of ruby both come from chromium ions in a crystal where $\Delta E$ differs). Haemoglobin is an iron(II) complex in which oxygen binds reversibly as a ligand, and carbon monoxide is toxic because it binds the same iron site far more strongly, an irreversible ligand substitution. Industrially, the catalytic power of iron, vanadium and nickel makes the Haber, Contact and hydrogenation processes economic, while colorimetric analysis of $\text{Cu}^{2+}$ or $\text{MnO}_4^-$ relies on the d-orbital colour CCEA asks you to explain.

Try this

Q1. State what is meant by a ligand. [2 marks]

• Cue. A molecule or ion that donates a lone pair to a metal ion to form a dative covalent bond.

Q2. Explain why transition metal complexes are coloured. [2 marks]

• Cue. The d-orbitals split, and electrons absorb visible light to move between the levels.