Nitrogen-bound 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.
Water is the preferred solvent ligand in the qualitative prediction. Its oxygen is a hard donor, it is less sterically demanding than ethanol, and it can occupy a crowded Eu(III) coordination sphere. Actual solvent occupancy also depends on activities and crystallization conditions, so a formula containing only the word solvent is not itself a crystallographic identification.
Two bidentate bipyridine ligands provide four N donors and three terminal NCS ligands provide three more. Adding one water gives and coordination number eight:Eight-coordinate arrangements, distorted between familiar ideal polyhedra, are plausible for a sterically crowded Eu(III) centre. A second water would give coordination number nine, which is not forbidden for a lanthanide; the requested value is a chemically motivated prediction, not something fixed by charge balance. Diffraction or a direct hydration-number measurement is needed to establish it uniquely.
An N-bound thiocyanate need not make an exactly linear Eu–N–C linkage. Eu(III) bonding is predominantly electrostatic, with weak directional covalent contributions from the shielded orbitals. There is consequently less energetic reward for enforcing a precisely collinear metal–donor arrangement than in a strongly directional transition-metal pi bond. In the crowded eight-coordinate sphere, bending the external NCS groups can reduce ligand–ligand repulsion while retaining favourable Eu–N distances. Angles of – are compatible with bent N coordination; they do not require changing the linkage to sulfur. This concerns the Eu–N–C angle, not bending the internal N–C–S skeleton.
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