VAPOR-PRESSURES AND STANDARD MOLAR ENTHALPIES OF SUBLIMATION OF 2 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 = 3 87.5 K and from T = 359.8 K to T = 377.8 K, respectively. From the tem perature dependence of the vapour pressure, the standard molar enthalp ies of sublimation at the mean temperature of the experimental tempera ture 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 es timated 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 = 2 98.15 K, Delta(cr)(g)H(m) degrees (298.15 K)/(kJ . mol(-1)), were esti mated as (128.6+/-0.9) and (129.3+/-1.2), respectively. From the stand ard molar enthalpies of formation of the gaseous complexes, the mean m olar (Fe-O) bond dissociation enthalpies were derived. (C) 1996 Academ ic Press Limited