Abstract
Standard enthalpies of formation of crystalline samples at 298.15 K were determined by solution calorimetry (oxidative hydrolysis), for ΔH<inf>f</inf><sup>⊖</sup>[Mo<inf>2</inf>(OPr<sup>i</sup>) <inf>6</inf>, c] = -(1661.8±9) kJ mol<sup>-1</sup> and ΔH<inf>f</inf><sup>⊖</sup>[Mo<inf>2</inf>(OPr<sup>i</sup>) <inf>8</inf>, c] = -(2292.5±10) kJ mol<sup>-1</sup>. The dissociation process [Mo<inf>2</inf>(OPr<sup>i</sup>)<inf>8</inf>, g] → 2 Mo(OPr<sup>i</sup>)<inf>4</inf>, g is calculated to be endothermic, ΔH ≈ 184 kJ mol<sup>-1</sup>. A bond-enthalpy/bond-length curve is tentatively proposed for Mo-Mo bonds. A similar curve for Mo-O bonds is given, to fit the limited thermochemical data available. It is concluded that the Mo=Mo bond in Mo<inf>2</inf>(OPr<sup>i</sup>)<inf>8</inf> contributes ca. 200-230 kJ mol<sup>-1</sup> to the binding in this molecule.