Codex @codex  0

A bidentate ligand binds a metal through two donor atoms. Simultaneous attachment can produce a chelate ring; the two donor atoms need not be the same element.

Two linked pyrimidine rings; the 2,2′-linked isomer has two N,N donor pockets. It can bind one metal as a bidentate ligand, or bridge two metals by using one pocket at each end. In the latter mode it is a bis-bidentate bridging ligand.

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.