Abstract
Enthalpies of reaction have been determined for a number of metal β-diketonates by means of the following reaction scheme: Mn+(sol) + nL-(sol) → MLn(sol) where L is a bidentate ligand. Subsequent determinations of the enthalpies of solution of all species permitted calculation of the following enthalpies of formation at 25°C of the crystalline complexes: [Al(dpm)3] ΔHf°(c) = -571.5±3.1 kcal/mol Hdpm=dipivaloylmethane; [Be(dpm)2] ΔHf°(c)= -386.5±2.0 kcal/mol; [Ni(dpm)2] ΔHf°(c)= -299.2±2.0 kcal/mol; [Al(tfacac)3] ΔHf°(c)= -892.7±3.1 kcal/mol Htfacac=trifluoro-acetylacetone; [Be(tfacac)2] ΔHf°(c)= -597.3±2.0 kcal/mol; [Alftrop)3]2 ΔHf°(c)= -316.96±0.66 kcal/mol Htrop=tropolone; [Be(trop)2] ΔHf°(c)= -208.56±0.42 kcal/mol; [Cu(trop)2] ΔHf°(c)= -102.47±0.42 kcal/mol; [Al(Metrop)3] ΔHf°(c)= -329.8±2.2 kcal/mol HMetrop=4-methyltropolone. Enthalpies of vaporization have been determined by a direct calorimetric method for the following β-diketones: Dipivaloylmethane Hdpm ΔHv=+14.23±0.03 kcal/mol. Trifluoroacetylacetone Htfacac ΔHv= +8.90±0.04 kcal/mol. Hexafluoroacetylacetone Hhfacac ΔHv +7.31±0.03 kcal/mol. Diisobutyrylmethane Hdidm ΔHv=+13.40±0.04 kcal/mol. Pivaloylpropionylmethane Hpiprm ΔHv=+13.59±0.03 kcal/mol. Isobutyrylpivaloylmethane Hibpm ΔHv=+13.80±0.05 kcal/mol. Using the appropriate auxiliary data, enthalpies of formation in the gaseous state have been calculated for the following compounds: [Al(dpm)3] ΔHf°(g) = -543.01±3.1 kcal/mol [Be(dpm)2] ΔHf°(g) = -361.8±2.1 kcal/mol [Ni(dpm)2] ΔHf°(g) = -262.2±10.2 kcal/mol [Al(tfacac)3] ΔHf°(g) = -867.2±3.1 kcal/mol [Be(tfacac)2] ΔHf°(g) = -574.3±2.1 kcal/mol [Al(trop)3] ΔHf°(g) = -287.0±5.0 kcal/mol [Be(trop)2] ΔHf°(g) = -183.6±5.0 kcal/mol [Cu(trop)2] ΔHf°(g) = -67.5±10.0 kcal/mol [Al(Metrop)3] ΔHf°(g) = -299.8±5.5 kcal/mol. From these, the metal-oxygen bond energies, E(M-O) have been determined: [Al(dpm)3] E(Al-O) = 61±3 kcal/mol [Be(dpm)2] E(Be-O) = 67±3 kcal/mol. [Ni(dpm)2] E(Ni-O) = 50±4 kcal/mol [Al(tfacac)3] E(Al-O) = 61±3 kcal/mol. [Be(tfacac)2] E(Be-0) = 67±3 kcal/mol. [Al(trop)3] E(Al-O) = 66±3 kcal/mol. [Be(trop)2] E(Be-O) = 71±3 kcal/mol. [Cu(trop)2] E Cu-O) = 42±4 kcal/mol [Al(Metrop)3] E(Al-O) = 64±3 kcal/mol. These results have been discussed, and in view of the uncertainties arising from a number of values which of necessity were estimated, a clear-cut relationship between bond strength and ligand parameters could not be demonstrated. An empirical order of bond strength has been derived from a consideration of the function ΔHf° [MLn](c) - nΔHf°HL (s or 1).