One novel cyanide-bridged mixed-valence {FeIII 2 (μ -CN) 4 FeII 2 } square cluster exhibits thermally-induced, reversible, complete spin transition behaviour at two out of four metal centers. • One novel cyano-bridged mixed-valence bimetallic {FeIII 2 (μ -CN) 4 FeII 2 } cluster has been prepared. • The square cluster exhibits thermally-induced, reversible, complete spin transition behavior. One novel cyano-bridged mixed-valence bimetallic {FeIII 2 (μ -CN) 4 FeII 2 } cluster, {[(Tp)FeIII(CN) 3 ] 2 [FeII(Py 2 N 2)] 2 }·(PF 6) 2 (1), has been synthesized (Tp = hydrotris(pyrazolyl)borate, Py 2 N 2 = N,N'-bis(2-pyridylmethyl)-N,N'-bis(4-benzyl)-1,2-ethanediamine). The molecular structure has been determined by single-crystal X-ray diffraction. Compound 1 exhibit temperature driven Fe(II) spin crossover behaviour (SCO) associated with the high-spin (HS) ↔ low-spin (LS) transition with the transition temperature of 130 K at two out of four metal centers. The evolution of HS and LS Fe(II) ions with temperature was followed by FT-IR, X-ray crystallography, the variable temperature magnetic susceptibility. [ABSTRACT FROM AUTHOR]