Variable-temperature X-ray structural investigation of {Fe[HC(3,5-Me(2)pz)(3)](2)}(BF4)(2) (pz = pyrazolyl ring): Observation of a thermally induced spin state change from all high spin to an equal high spin-low spin mixture, concomitant with the onset of nonmerohedral twinning

The complex {Fe[HC(3,5-Me(2)pz)(3)](2)}(BF4)(2) (pz = pyrazolyl ring) under goes a phase transition that occurs concomitantly with a thermally induced spin conversion between the high-spin (HS. S = 2) and low-spin (LS, S = 0) states. Above 204 K the compound is completely HS with the structure in the C2/c space group with Z = 4. A crystal structure determination of this pha se was performed at 220 K yielding the cell constants a = 20.338(2) Angstro m, b = 10.332(1) Angstrom, c = 19.644(2) Angstrom, beta = 111.097(2)degrees , and V = 3851.5(6) Angstrom (3). There is one unique iron(II) site at this temperature. Below 206 K the compound converts to a 50:50 mixture of HS an d LS, The radical change in the coordination sphere for half of the iron(II ) sites, most notably a shortening of the Fe-N bond distances by ca. 0.2 An gstrom, that accompanies this magnetic transition causes a phase transition . The crystal system changes from C-centered monoclinic to primitive tricli nic with Z = 2 with two half-molecules on independent inversion centers. A crystal structure determination was performed at 173 K in space group P (1) over bar with a 10.287(2) Angstrom, b = 11.355(3) Angstrom, c 18.949(4) An gstrom, alpha = 90.852(4)degrees, beta = 105.245(4)degrees, gamma = 116.304 (4)degrees, and V = 1892.3(8) Angstrom (3). All specimens investigated belo w the phase transition temperature were determined to be nonmerohedral twin s. Temperature cycling between these two forms does not appear to degrade c rystal quality. previous magnetic susceptibility measurements indicate a se cond, irreversible increase in the magnetic moment the first time the cryst als are cooled below 85 K. A crystal structure determination at 220 K of a specimen precooled to 78 K was not significantly different from those not c ooled below 220 K.