Pressure-Tuning of Magnetism and Linkage Isomerism in Iron(II) Hexacyanochromate

A pressure-induced linkage isomerization of the cyanide anion has been observed in single crystals of a chromium(III)−iron(II) Prussian blue analogue of formula K_0.4Fe₄[Cr(CN)₆]_2.8□_1.2·16H₂O (1). Upon application of pressure in the 0−1200 MPa range, the cyanide ligand rotates and becomes C-bonded to the iron(II) cations, leading to a stabilization of their diamagnetic low-spin states. The result is a decrease of magnetization and magnetic ordering temperatures from T_C = 19 K at ambient pressure to 13 K at 1200 MPa. The initial magnetic properties can be restored on pressure release. The reversible movement of cyanide in the solid state can be exploited as a switch of the magnetic interaction at the molecular level.