An unprecedented hetero-bimetallic three-dimensional spin crossover coordination polymer based on the tetrahedral [Hg(SeCN)4]2− building blockElectronic supplementary information (ESI) available: Crystal data and details of diffraction experiments. The selected bond lengths and angles. View of the unit cell of compounds 1 and 2, along the [100] direction and two consecutive arrays of layers viewed along the [001] direction. Two orthogonal views of the same structural motif showing four fragments

Self-assembly of octahedral Fe II 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 Hg II -Fe II 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 T 1/2 = 107.8 K and complete photoconversion of the low spin state into a metastable high-spin state (LIESST effect) with T LIESST = 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. A new non-Hoffmann type of hetero-bimetallic Hg II -Fe II spin-crossover coordination polymer has been structurally and magnetically investigated.