The suitability of the system [Fe(4,4'-bipy)(H2O)(2)(NCX)(2)].(4,4'-bipy), where 4,4'-bipy stands for 4,4'-bipyridine and X = S (1) and Se (2), as a precursor for the synthesis of new polymeric spin-crossover compounds has been studied. The reaction of 1 or 2 with bt (2,2'-bithiazoline) afforded the polymeric compounds of formula [Fe(4,4'-bipy)(bt)(NCX)(2)] (X = S (3), Se (4)). Compounds 3 and 4 are isostructural, but only the crystal structure of 3 has been fully determined. It crystallizes in the orthorhombic system, Fdd2 space group, Z = 24, with a = 38.962(8) Angstrom, b = 11.545(2) Angstrom, c = 30.889(6) Angstrom, V = 13895(5) Angstrom(3). The structure consists of linear chains constituted by trans-4,4'-bipy linked iron(II) ions; two cis equatorial positions are occupied by two pseudohalide ligands, and the remaining positions are filled by the bidentate bt ligand. Investigation of their magnetic properties and Mossbauer spectra has revealed the occurrence of a low-spin (LS) <----> high-spin (HS) conversion involving 12% (3, S) and 20% (4, Se) of the Fe(II) ions. The thermal variation of the HS fraction is gradual with onset temperatures as low as 60 K. A theoretical approach based on the Ising-like model, completed with molecular vibrations, through harmonic oscillators, fits the data successfully, leading to an energy gap of 65 cm(-1) (3) and 86 cm(-1) (4) between the lowest LS and HS levels, and an average vibration frequency COLS of 382 cm(-1) (3) and 365 cm(-1) (4) in the LS state. The ca. 1.05 omega(LS(3))/omega(LS(4)) ratio is close to the ca. 1.09 Se/S molar mass ratio. The simple electrovibrational Ising-like model permits us to explain, for the first time, a mass effect through the molecular vibrations in a spin-crossover complex that is in the unusual situation of equienergy among the HS and LS states.