Spin transition centres linked by the nitroprusside ion. The cooperative transition in bis(2,6-bis(pyrazol-3-yl)pyridine)iron(II) nitroprusside

The complex [Fe(3-bpp)(2)][Fe(CN)(5)(NO)] (3-bpp = 2,6-bis(pyrazol-3-yl)pyridine) undergoes an abrupt, thermally-induced (1)A(1) <-> T-5(2) spin transition associated with a narrow (similar to3 K) hysteresis loop (T(1/2)down arrow = 181 K; T(1/2)up arrow = 184 K). The transition is accompanied by a change in crystallographic space group (P4/ncc at 294 K; Pnca at 100 K) along with a change in the average Fe-N distance of 0.21 Angstrom. The complex cation and anion are linked by hydrogen bonding involving each of the four pyrazolyl-NH moieties of each complex cation and the four equatorial cyano groups of each anion, resulting in layers of 4,4-nets. The hydrogen bonding remains essentially the same in both phases but the relative orientations of the cations and anions change significantly to allow for the change in the dimensions of the complex cation, accounting for the cooperativity associated with the spin change.