High temperature microcalorimetric studies of the thermal decomposition and iodination of polynuclear carbonyls of Fe, Co, Ru, Rh, Re, Os and Ir

Abstract

From measurements of the enthalpies of thermal decomposition (and also in some cases of the enthalpies of reaction with iodine vapour) at elevated temperatures, the standard enthalpies of formation, ΔH^°_f, at 298 K of the following crystalline metal carbonyls have been obtained (in kJ mol^–1): Fe₂(CO)₉=–1410±12; Fe₃(CO)₁₂=–1849±16; Co₂(CO)₈=–1209±8; Co₄(CO)₁₂=–1845±16; Ru₃(CO)₁₂=–1920±20; Rh₄(CO)₁₂=–(1820±12); Rh₆(CO)₁₆=–(2029); Re₂(CO)₁₀=–1653±20; Os₃(CO)₁₂=–(1749±20); Ir₄(CO)₁₂=–1820±16, Estimates of the unknown values ΔH_sub(298 K) for the crystalline compounds have been used to derive the values ΔH^°_f(g); the latter were converted into ΔH values for the total enthalpies of disruption of the gaseous carbonyls into metal atoms and CO gaseous molecules. These ΔH values have in turn been reduced to individual enthalpy contributions from the various metal-carbonyl and metal-metal bonds in the molecule. Terminal metal-carbonyl bond enthalpies increase both with atomic number, and with enthalpy of atomization of the metal. The enthalpy contribution from a metal-carbon bond in a bridging M—CO—M is of the order one half of the corresponding terminal M—CO linkage. The bond enthalpy contribution from M—M in the polynuclear carbonyls of Fe and Co is found to be approximately 2/3 of that from the M—CO terminal bonds.