The properties of transition metals, variable oxidation states, complex ions and ligands, the origin of colour, catalysis, and ligand substitution reactions.

What this topic is asking

Edexcel Topic 14 wants you to explain the characteristic properties of transition metals, describe complex ions and ligands, account for colour using d-orbital splitting, explain homogeneous and heterogeneous catalysis, and describe ligand substitution reactions, including the colours observed.

The answer

Properties of transition metals

Variable oxidation states arise because the $3\text{d}$ and $4\text{s}$ sub-shells are close in energy, so successive electrons are lost with similar energies. Iron, for example, forms both $\text{Fe}^{2+}$ and $\text{Fe}^{3+}$; manganese ranges from $+2$ up to $+7$ in $\text{MnO}_4^-$. Scandium ($\text{Sc}^{3+}$, empty $3\text{d}$) and zinc ($\text{Zn}^{2+}$, full $3\text{d}$) are excluded because they cannot form an ion with a partially filled d sub-shell.

Complex ions and ligands

Replacing monodentate ligands with a multidentate one increases the entropy of the system (more free particles are released), which is the basis of the chelate effect that makes multidentate complexes especially stable.

Colour

A larger splitting absorbs higher-energy (shorter-wavelength) light. Because the absorption depends on the ligands, changing the ligand (as in a substitution) usually changes the colour, which is why complexes are used in colorimetry to measure concentration.

Catalysis

Transition metals catalyse reactions in two ways:

• Homogeneous (catalyst and reactants in the same phase): the metal uses its variable oxidation states to provide an alternative route, for example $\text{Fe}^{2+}/\text{Fe}^{3+}$ catalysing the reaction between $\text{S}_2\text{O}_8^{2-}$ and $\text{I}^-$.
• Heterogeneous (different phase): reactants adsorb onto the metal surface, where bonds weaken and react, then products desorb, for example iron in the Haber process and nickel in hydrogenation.

Ligand substitution

Ligand substitution replaces one ligand with another, sometimes changing the colour and the coordination number. Adding excess ammonia to pale blue $[\text{Cu(H}_2\text{O})_6]^{2+}$ gives the deep blue $[\text{Cu(NH}_3)_4(\text{H}_2\text{O})_2]^{2+}$; adding concentrated $\text{HCl}$ gives the yellow tetrahedral $[\text{CuCl}_4]^{2-}$ (a change of coordination number from 6 to 4).

Examples in context

Example 1. Haemoglobin and carbon monoxide poisoning. Haemoglobin contains an iron(II) ion in a multidentate haem ring, with a sixth coordination site that binds oxygen reversibly for transport. Carbon monoxide is a stronger-field ligand and binds far more strongly at that site by ligand substitution, blocking oxygen transport. This is transition-metal complex and ligand-substitution chemistry from Topic 14 applied to a biological molecule of life-or-death importance.

Example 2. Determining copper by colorimetry. Because the colour of a copper-ammonia complex depends on the concentration of the absorbing species, an analyst can add excess ammonia to a copper salt to form the deep blue $[\text{Cu(NH}_3)_4(\text{H}_2\text{O})_2]^{2+}$ and measure its absorbance in a colorimeter. Comparison with a calibration graph gives the copper concentration. This routine technique relies directly on d-orbital splitting producing a measurable colour.

Try this

Q1. Define a transition metal. [1 mark]

• Cue. A d-block element that forms at least one stable ion with a partially filled d sub-shell.

Q2. Explain why aqueous copper(II) ions are blue. [3 marks]

• Cue. The d-orbitals split; an electron absorbs visible light to be promoted between the levels, and the complementary colour (blue) is transmitted.