A steep one-step [HS-HS] to [LS-LS] spin transition in a 4,4'-bipyridine linked one-dimensional coordination polymer constructed from a pyrazolato bridged Fe(II) dimer.

The variable temperature magnetic susceptibility, X-ray crystallography, and Mössbauer and Raman spectra of a new dinuclear complex-based one-dimensional coordination polymer [[Fe(II)2(NCS)2(mu-bpypz)2](micro-4,4'-bpy)].MeOH demonstrated a steep one-step [HS-HS] to [LS-LS] spin transition.