Melting of group VI transition metal hexacarbonyls: thermodynamic parameters

Abstract

The melting of group VI transition metal hexacarbonyls in sealed, evacuated glass ampoules has been studied by differential scanning calorimetry. The thermodynamic parameters are: Cr(CO)₆:ΔH°= 5.6 ± 0.1 kcal mol^–1, ΔS°= 13.2 cal°C^–1(m.p. 150 ± 2°C), Mo(CO)₆:ΔH°= 6.4 ± 0.1 kcal mol^–1, ΔS°= 15.3 cal°C^–1 mol^–1(m.p. 146 ± 2°C), W(CO)₆:ΔH°= 6.5 ± 0.1 kcal mol^–1, ΔS°= 14.8 cal°C^–1 mol^–1(m.p. 166 ± 2°C)

These values were combined with the reported sublimation data and the enthalpies of vaporization of the three metal carbonyls calculated. Using the reported enthalpies of sublimation and the calculated enthalpies of vaporization, the solid-gas and liquid-gas phase diagrams have been obtained. From the phase diagrams the boiling points of Mo(CO)₆ and W(CO)₆ have been calculated to be 160.8 and 184.2°C, respectively.