CAIE A-Level Chemistry 28.1.6 Formation of Transition-Metal Complexes

Practise explaining complex-ion formation by donation of ligand lone pairs into accessible vacant metal d orbitals.

Exam points

identify an energetically accessible vacant d orbital on the central transition-metal ion
show a ligand donating a lone pair into that orbital to form a dative covalent bond
recognise that several such metal–ligand bonds produce a complex ion with a defined coordination number

[Maximum number: 1]

Titanium is a transition element in Period 4. It is commonly found as $\mathrm{TiO}_{2}$ in minerals.

Acidified $\mathrm{Ti}^{3+}(\mathrm{aq})$ reacts with oxygen dissolved in water as shown.

4 \mathrm{Ti}^{3+}+\mathrm{O}_{2}+2 \mathrm{H}_{2} \mathrm{O} \rightarrow 4 \mathrm{TiO}^{2+}+4 \mathrm{H}^{+} \quad \Delta G^{\ominus}=-436.1 \mathrm{~kJ} \mathrm{~mol}^{-1}

The standard reduction potential, $E^{\ominus}$ , of $\mathrm{O}_{2}+4 \mathrm{H}^{+}+4 \mathrm{e}^{-} \rightleftharpoons 2 \mathrm{H}_{2} \mathrm{O}$ is +1.23 V .

When aqueous citrate ions, $\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}_{7}{ }^{3-}$ , are added to $\mathrm{Ti}^{3+}(\mathrm{aq})$ , the $\left[\mathrm{Ti}\left(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}_{7}\right)_{2}\right]^{3-}(\mathrm{aq})$ complex forms.

Explain, in terms of d-orbitals, why $\mathrm{Ti}^{3+}$ is able to form complex ions.

$\mathrm{Ti}^{3+}$ empty / vacant d orbitals can form dative bonds / accept a lone pair from a ligand
OR $\mathrm{Ti}^{3+}$ has vacant d-orbitals which are energetically accessible

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28.1.6—Why transition elements form complex ions question 1