Poly(ethylene oxide) methyl ether 4H-1,2,4-triazol-4-yl (1) has been synthesized as a novel macro-ligand. ESI-MS (electrospray ionisation mass spectrometry), MALDI-TOF (matrix-assisted laser desorption ionisation time-of-flight) MS as well as NMR spectroscopy were used for the accurate determination of number-average molecular weights ((M) over bar (n)), the crucial parameter for subsequent synthesis of metal-polymer complexes of four different samples 1a-d. Reaction of polymer la with [Fe(H2O)(6)](nps)(2) (nps = 3-nitrophenylsulfonate) gave [Fe(1a)(3)](nps)(2), a bistable metal-polymer complex that exhibited low-spin (LS) to high-spin (HS) transitions around T-1/2=257 K, accompanied by a change in colour from violet (LS) to slightly yellow (HS).