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Each thiocyanate is a three-metal bridge, ${\mu }_3$-$\kappa N:\kappa S:\kappa S$: its nitrogen donates to one metal and its sulfur donates to two other metals. Thus the coordination ligand is ambidentate, but the structure uses both ends and shares the sulfur donor. Each metal receives two nitrogen and four sulfur donors, giving an approximately octahedral $\mathrm{M}{\mathrm{N}}_2{\mathrm{S}}_4$ coordination sphere. Six donor contacts per metal are consistent with two coordination ligands per metal because each coordination ligand contributes three contacts across the lattice. Sharing these contacts produces a coordination polymer within each layer; the interlayer Van der Waals force does not add coordination bonds. The index ${\mu }_3$ counts metal centres, whereas a hapticity label would count adjacent coordinated coordination ligand atoms and would not describe this connectivity correctly.^[ref]

Distinguish the original thiirane sulfur $S_o$ from the sulfur $S_n$ supplied by thiocyanate. sulfur attack by ${}^{-}{\mathrm{S}}_{\mathrm{n}}-\mathrm{C}\equiv \mathrm{N}$ at one thiirane carbon opens its C–$S_o$ bond with inversion. This produces a thiolate at the neighbouring carbon and a C–$S_n$–CN group at the attacked carbon. The OH-bearing remote carbon is unaffected.

The original thiolate $S_o^{-}$ now attacks the nitrile carbon intramolecularly. Shift one C≡N pi pair onto nitrogen. The resulting cyclic imidodithiocarbonate has the five-membered sequence ring carbon–$S_o$–C(=N${}^{-}$)–$S_n$–adjacent ring carbon; its two carbon atoms are also part of the original six-membered carbocycle, exactly giving the hinted bicyclic intermediate.

Collapse can reform C≡N while breaking the nitrile-carbon–$S_n$ bond. This transfers the CN group to $S_o$ and generates $S_n^{-}$ as the thiolate on the other ring carbon. That thiolate attacks the remaining C–$S_o$ bond from the back, expelling ${}^{-}{\mathrm{S}}_{\mathrm{o}}-\mathrm{C}\equiv \mathrm{N}$ and reclosing the thiirane. Each thiirane carbon has undergone one inversion, so the new sulfur bridge lies on the opposite face; the OH configuration is retained. The thiocyanate sulfur has entered the ring and the original ring sulfur has left as thiocyanate. This atom-labelled exchange mechanism explains how the same elemental formula can conceal sulfur replacement; isotope labelling would be needed to test that exchange experimentally. A single epimerization at the OH carbon would give the wrong stereochemical outcome.

With oxide charge $-2$ and four thiocyanates each $-1$, $x-2-4=-2$ gives V(IV), $d^1$. Counting the terminal oxo as $X_2$ and each thiocyanate as X givesThe expected structure is square-pyramidal molecular geometry, with the short V=O unit at the apex and the four thiocyanate donors in the basal plane. The remaining d electron can occupy an approximately basal $d_{xy}$ orbital that avoids the strongest donor directions and the oxo pi-antibonding orbitals. This is a high-oxidation-state oxovanadium complex; reaching 18 electrons is not its governing stability criterion. The displayed $\mathrm{SCN}$ composition alone does not establish N versus S linkage, and that choice does not change this count. The stated 13-electron result consistently uses a four-electron $\mathrm{M}=\mathrm{O}$ convention; counting extra oxide pi-donor interactions as additional ligand electrons requires a different convention throughout.

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.

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 $4f$ 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 $150^{\circ }$–$160^{\circ }$ 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.

The bipyrimidine ligand has two distinct N,N-chelating pockets. It can use one pocket as a terminal bidentate ligand, or use both to bridge two Eu centres. Take all three nitrates per Eu to be bidentate O,O donors.

A consistent structure for B is $\mathrm{Eu}(\mathrm{N}{\mathrm{O}}_3{)}_3(\mathrm{bpm}{)}_2$, with two terminal, singly chelating bpm ligands. Six nitrate oxygens plus four bpm nitrogens give coordination number ten. The unused pocket of each terminal bpm points away from that Eu centre.

For C, use a dimer $\mathrm{E}{\mathrm{u}}_2(\mathrm{N}{\mathrm{O}}_3{)}_6(\mathrm{bpm}{)}_3$. Place one bpm between the metals, chelating each Eu through a different N,N pocket. Add one terminal bidentate bpm to each Eu and three bidentate nitrates to each Eu. Each metal again has $2+2+6=10$ donor atoms, while the overall Eu:bpm ratio is $2:3$. In this model all four nitrogens of the bridging bpm are used, but only two of each terminal bpm are used.

Both structures have their accessible coordination sites occupied by multidentate ligands, and the chelate effect favours retention of those donors over adding or exchanging a solvent molecule. This accounts for the absence of coordinated solvent while allowing a higher coordination number than the thiocyanate example. Stoichiometry alone does not exclude polymeric alternatives or different nitrate binding modes; the monomer/dimer pair is an explicit internally consistent prediction.

thiocyanate can coordinate through nitrogen, sulfur, or both ends, and can bridge several metal centres. A linkage label specifies donor atoms, whereas the bridging index ${\mu }_n$ specifies the number of connected metals. A short C–N bond and long C–S bond favour an $\mathrm{N}\equiv \mathrm{C}-{\mathrm{S}}^{-}$ description; greater C–S and smaller C–N bond orders give more ${}^{-}\mathrm{N}=\mathrm{C}=\mathrm{S}$ character. Internal distances alone need not determine how many metals are attached.

Sequential ring opening, five-membered cyclic imidodithiocarbonate formation, cleavage and reclosure can exchange the thiirane sulfur with the thiocyanate sulfur. Backside substitution at both ring carbons reverses the thiirane face while leaving remote stereocentres intact.

A ${\mu }_3$ thiocyanate connecting three metal centres. In layered first-row divalent metal thiocyanates, its nitrogen coordinates one metal and its sulfur coordinates two others, giving an $\mathrm{M}{\mathrm{N}}_2{\mathrm{S}}_4$ octahedral coordination environment.