Low-temperature vapor pressures of W-, Cr-, and Co-carbonyls

Abstract

Low-temperature vapor pressures of three metal carbonyls—W(CO) ₆, Cr(CO)₆, and Co₂(CO)₈—were determined using the gravimetric torsion effusion method. The vapor pressures of W(CO)₆ ranged from 10-⁵ to 10-⁴ kPa in a temperature range of 265 to 288 K, and those of Cr(CO)₆ ranged from 10-₄ to 10-³ kPa in a temperature range of 266 to 272 K. Both of these monomer carbonyls were virtually free of disproportionation during vaporization, and the measured molecular weights of these compounds were very close to the actual monomer values. The vaporization behavior of Co₂(CO)₈ exhibited complex behavior, and it decomposed to cobalt metal at room temperature, in vacuo. The decomposition of (dimer) Co₂(CO)₈ to metallic Co did not occur below room temperature at ~278 K, but the dimer disproportionated to other molecular forms, suggested by the differences in measured molecular weight of vaporizing gas species of 229 g/mol as compared to an actual value of 342 g/mol. It is proposed that the dimer, Co₂(CO)₈, undergoes partial disproportionation to tetramer [Co₄(CO)₁₂], monomer [Co₂(CO)₈], and CO gas. The vapor pressures of each of the above-mentioned carbonyls, the average molecular weights of the effusing gases, proposed disproportionation mechanisms for cobalt carbonyl, equilibrium constants for the vaporization reactions, their enthalpies, entropies, and Gibbs energies are presented.