METAL-LIGAND BOND-ENERGIES AND SOLVATION ENERGIES FOR GAS-PHASE TRANSITION-METAL TRIS(ACETYLACETONATE) COMPLEXES AND THEIR NEGATIVE-IONS

The gas-phase bond disruption enthalpies and solvation free energies for several tris(acetylacetonate) transition metal(III) complexes (M(acac)3) and their metal(II) anions are derived from the results of Fourier transform ion cyclotron resonance (FTICR) studies of electron attachment to the complexes and the acetylacetonyl radical (acac). New values are reported for the average M-O bond disruption enthalpies, both homolytic (DELTA-H-degrees-hom(M-O)) and heterolytic (DELTA-H-degrees-het(M-O)), for M(acac)3 with M = Cr, Mn, Fe, and Co. The values of DELTA-H-degrees-hom(M-O) are reevaluations of earlier results obtained from reaction calorimetry and are based on new experimental data for the electron attachment energy of acetylacetonyl radical (C5H7O2.) determined by Fourier transform mass spectrometry (-58 +/- 3 kcal mol-1) and a new value derived for the homolytic O-H bond dissociation enthalpy in the enol form of acetylacetone (88 +/- 6 kcal mol-1). From the gas-phase electron attachment free energies for a series of M(acac)3 complexes (DELTA-G-degrees-a(M(acac)3,g)) for M = Cr, Mn, Fe, Co, a thermodynamic cycle is used to obtain average DELTA-H-degrees-het(M-O) values for the corresponding gas-phase anionic M(acac)3- complexes. For the M(acac)3(g) complexes of the metals Cr, Mn, Fe, and Co, respectively, the following mean bond disruption enthalpies (kcal mol-1) are obtained: DELTA-H-degrees-hom(M-O) = 45 +/- 3, 33 +/- 3, 36 +/- 3, 33 +/- 3; DELTA-H-degrees-het(M-O) = 224 +/- 5, 222 +/- 5, 217 +/- 5, 228 +/- 5. For the M(acac)3-(g) anions the following mean bond disruption enthalpies are obtained for M = Cr, Mn, Fe, Co, respectively: DELTA-H-degrees-hom(M-O) = 46 +/- 3, > 43, 43 +/- 3, 38 +/- 4; DELTA-H-degrees-het(M-O) = 108 +/- 8, 103 +/- 8, 107 +/- 8, 107 +/- 8. The DELTA-G-degrees-a(M(acac)3,g) values for several complexes combined with estimates of single electrode potentials for the same M(acac)3(0/-) couples in solution are used to obtain the change in solvation free energies (DELTA-DELTA-G-degrees-solv) for these couples in acetonitrile. Values of DELTA-DELTA-G-degrees-solv obtained are in the range of -50 +/- 5 kcal mol-1 for couples involving first-row transition metals. These assessments of bond energies and solvation energies based on gas-phase electron attachment energies lead to a complete quantitative interpretation of the observed solution electrode potentials for the M(acac)3 complexes studied.