Volatility and chemical stability of chromium, molybdenum, and tungsten hexacarbonyls

The sublimation vapor pressures of three metallic (group 6) hexacarbonyls, Cr(CO)₆, Mo(CO)₆, and W(CO)₆, were measured using the Knudsen mass-loss effusion method. The standard (p^o = 0.1 MPa) molar enthalpies, entropies, Gibbs energies, and heat capacity difference between the gas and solid state, at T = 298.15 K, were derived from the experimental results combined with selected literature ones covering a wide range of temperature. The temperatures and molar enthalpies of fusion of those compounds were measured using differential scanning calorimetry. The thermodynamic stability of the hexacarbonyls was evaluated taking into account the standard Gibbs energies of formation in the crystalline and gaseous phases. Gas-phase absolute entropies, heat capacities, and enthalpies of formation of the three compounds studied as well as the bond distances M–C and C–O were calculated.