Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2021 / Paper 1 / 10 / c / i

Nitrogen-bound $\mathrm{Eu}-\mathrm{NCS}$ is favoured. Eu(III) is a small, highly charged hard Lewis acid, whereas the nitrogen end of thiocyanate is harder than its more polarizable sulfur end. The hard and soft acids and bases argument therefore predicts N coordination, together with coordination through both nitrogen atoms of each bipyridine.