COOPERATIVE SPIN TRANSITIONS IN IRON(II) DERIVATIVES OF 1,2,4-TRIAZOLE

Complexes of stoichiometry Fe(trzH)2(trz)X, where trzH is 1,2,4-triazo le and X = BF4, ClO4, PF6, have been prepared. All three salts display , temperature-induced singlet (1A1) reversible quintet (5T2) transitio ns which are generally discontinuous and associated with a broad hyste resis loop. The salts are strongly thermochromic, being white above an d pink-lilac below the transition region. Several samples of each salt were obtained and the detailed behaviour varied somewhat from one sam ple to another. For the fluoroborate and perchlorate salts the transit ion is centred above room temperature while for the hexafluorophosphat e salt it occurs below room temperature. The perchlorate salt was also isolated in a form which showed a continuous transition but the natur e of its electronic properties changed as the sample aged. In one form of the hexafluorophosphate salt the hysteresis loop spans room temper ature and it has been possible to characterize both the singlet and th e quintet state species at that temperature. For all three salts the t ransition has been characterized by measurement of the temperature dep endence of the magnetism and Mossbauer and electronic spectra. These m easurements establish that the complexes all contain an FeN6 coordinat ion core and this must be achieved through bridging of the triazole un its.