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The Hg–S interaction is expected to be stronger than Hg–N by the hard and soft acids and bases argument. $\mathrm{H}{\mathrm{g}}^{2+}$ is a soft, readily polarizable Lewis acid. The sulfur end of thiocyanate is a softer and more polarizable donor than its nitrogen end, so it better matches mercury's electronic response and covalent bonding preference.

Nitrogen being more electronegative does not by itself make it the preferred donor to a soft metal. Charge concentration and polarizability are distinct properties. The extended solid can still contain Hg–N contacts that complete the bridging network; the prediction concerns their relative bonding preference, not the absence of N coordination.^[ref]

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.

Ralph G. Pearson coauthored research on hard and soft acids and bases.^[ref]

A qualitative matching framework for Lewis acids and bases. Hard species favour interactions with compact, less polarizable partners, whereas soft species favour more polarizable partners. It is not an ordering based solely on atomic electronegativity.