Vapour pressures and standard molar enthalpies of sublimation of two crystalline iron(III) beta-diketonates. The mean molar (Fe-O) bond-dissociation enthalpies

The simultaneous torsion and mass-loss effusion technique was used to measure the vapour pressure of the crystalline complexes of iron(III) with pentane-2,4-dione (HACAC) and with 2,2,6,6-tetramethylheptane-3,5-dione (HDPM), as a function of temperature, from T = 369.3 K to T = 387.5 K and from T = 359.8 K to T = 377.8 K, respectively. From the temperature dependence of the vapour pressure, the standard molar enthalpies of sublimation at the mean temperature of the experimental temperature ranges, were derived for both complexes. The mean values obtained from both techniques are: for Fe(ACAC)(3), Delta(cr)(g)H(m) degrees (377.8 K)/(kJ . mol(-1)) = (124.6+/-0.9); and for Fe(DPM)(3), Delta(cr)(g)H(m) degrees(377.8 K)/ (kJ . mol(-1)) = (125.8+/-0.9). Using the estimated value of Delta(cr)(g)C(p,m)degrees = -50 J . K-1. mol(-1), for both complexes, the standard molar enthalpies of sublimation at T = 298.15 K, Delta(cr)(g)H(m) degrees (298.15 K)/(kJ . mol(-1)), were estimated as (128.6+/-0.9) and (129.3+/-1.2), respectively. From the standard molar enthalpies of formation of the gaseous complexes, the mean molar (Fe-O) bond dissociation enthalpies were derived. (C) 1996 Academic Press Limited