An unprecedented hetero-bimetallic three-dimensional spin crossover coordination polymer based on the tetrahedral [Hg(SeCN)] building block.

Self-assembly of octahedral FeII ions, trans-1,2-bis(4-pyridyl)ethane (bpe) bridging ligands and [Hg(XCN)4]2- (X = S (1), Se (2)) tetrahedral building blocks has afforded a new type of hetero-bimetallic HgII-FeII spin-crossover (SCO) 3D 6,4-connected coordination polymer (CP) formulated {Fe(bpe)[Hg(XCN)4]}n. For X = S (1), the ligand field is close to the crossing point but 1 remains paramagnetic over all temperatures. In contrast, for X = Se (2) the complex undergoes complete thermal induced SCO behaviour centred at T1/2 = 107.8 K and complete photoconversion of the low spin state into a metastable high-spin state (LIESST effect) with TLIESST = 66.7 K. The current results provide a new route for the design and synthesis of new SCO functional materials with non-Hoffmann-type structures.