Microcalorimetric studies. Thermal decomposition and iodination of metal carbonyls

Abstract

The Calvet high temperature microcalorimeter was used to measure the enthalpies of thermal decomposition and the enthalpies of reaction with iodine vapour, of carbonyls of Mo, Mn, and Cr. ΔH^°_f(c) values for Mo(CO)₆, Mn₂(CO)₁₀, Cr(CO)₆, Mn(CO)₅Cl and Mn(CO)₅Br were obtained. These studies have confirmed the literature value for ΔH^°_f(Mn₂(CO)₁₀, c), but disagree with the currently accepted value for ΔH^°_f(Cr(CO)₆, c). The bond dissociation energies D[(CO)₅Mn—Cl] and D[(CO)₅Mn—Br] were evaluated at 73 and 61 kcal mol^–1 respectively: these values are ca. 20 kcal mol^–1 less than the mean bond dissociation energies, D(Mn—Cl) and D(Mn—Br), in MnCl₂(g) and MnBr₂(g), and ca. 15 kcal mol^–1 less than the dissociation energies in the diatomic molecules MnCl, MnBr.