THE THIRD LAW ENTROPY AND STRUCTURE OF IRON PENTACARBONYL

The heat capacity of solid and liquid iron pentacarbonyl has been measured from 20° to 290°K. The heat of fusion was also measured and found to be 3161 ± 3 cal/mole. From an investigation of the change of temperature of the sample during melting the triple point of iron pentacarbonyl was estimated to be 253.15°K and the purity of the sample 99.9 mole%. An anomalous absorption of energy amounting to 4.8 cal/mole was observed at about 230°K; this is shown to be due to an unknown impurity in the sample. The vapor pressure of the liquid was measured in the vicinity of room temperature and from these results the heat of vaporization at 25 °C was estimated to be 9.1 ± 0.1 kcal/mole.The entropy of iron pentacarbonyl as an ideal gas at 1 atm pressure and 25 °C computed from the thermal measurements is 106.4 ± 0.8 cal/mole deg. Comparison of this quantity with values calculated from spectroscopic data indicates that the iron pentacarbonyl molecule has the form of a trigonal bipyramid rather than that of a square pyramid.