Appearance Potentials, Bond Dissociation Energies, and Heats of Formation

APPEARANCE POTENTIALS, BOND DISSOCIATION ENERGIES, AND HEATS OF FORMATION 

The appearance potentials for molecular ions (ionization potentials) and for fragment ions formed in the mass spectra of metallocenes and related compounds are listed in Table XIII. These appearance potentials have been used to calculate bond dissociation energies and heats of formation of organometallic compounds, but the results obtained must be treated cautiously because the appearance potentials of fragment ions include excess energy due to excited species. The values obtained for the heats of formation are best considered as upper limits, rather than precise determinations. The extent to which energy due to excited states can contribute 

The values obtained for the ionization potentials of organometallic molecules are much lower than the ionization potentials of the ligands (Table XIV) and are much closer to the ionization potentials of the central metal atom (Table XV), indicating that ionization subsequent to electron impact involves an electron associated with the metal atom. 

In several cases shown in Table XV, the ionization potential of the molecule is 1-1.5 eV lower than that of the metal atom, suggesting a high stability of the molecular ion with respect to neutral species, and this is more noticeable with the second- and third-row transition metals such as molybdenum, rhenium, tungsten, and osmium. 

The value of 6.4 eV is in between the ionization potentials of the complex and the free metal atom and is probably due to a C5H6Cr fragment formed by thermal decomposition in the spectrometer. 

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