Complexes of dimolybdenum(II) and dichromium(II) with 2-hydroxy-6-methylpyridine (Hmhp) and m-dimethoxybenzene (HDMP). Enthalpies of formation of [Mo2(mhp)4], [Cr2(mhp)4], [Mo2(mhp)2(OAc)2] and [Cr2(DMP)4]

Abstract

From calorimetric measurements at 25 °C of the enthalpies of oxidative hydrolysis in solution, the standard enthalpies of formation of the following crystalline compounds were obtained (values in kJ mol^–1): [Mo₂(mhp)₄]=–(754.0 ± 9); [Mo₂(mhp)₂(OAc)₂]=–(1366.8 ± 12); [Cr₂(mhp)₄]=–(948.2 ± 9); and [Cr₂(DMP)₄]=–(961 ± 22). The enthalpies of formation in the gaseous state and the enthalpies of atomization (ΔH^°_a) of these compounds were evaluated, using measured (or estimated) enthalpies of sublimation.

An attempt is made to quantify the contributions to ΔH^°_a from the metal–metal, metal–oxygen, metal–nitrogen and metal–carbon bonds in these molecules. The analysis is based on provisional ‘bond enthalpy/bond length’ curves for Mo—Mo and Cr—Cr bonds. These in turn are used with available thermochemical and structural data to construct bond enthalpy/bond length curves for Mo—O and Cr—O bonds. The values obtained for E(Cr—N) in [Cr₂(mhp)₄], and for E(Cr—C) in [Cr₂(DMP)₄] and [Cr(CO)₆] indicate that at the same bond length, E(Cr—O) > E(Cr—C) > E(Cr—N).