Periodic trends in gas phase MH and MC bond energies

Abstract

Thermochemistry of simple transition metal species as determined by guided ion beam mass spectrometry is reviewed. Compounds which are discussed include metal hydrides (both ionic and neutral), metal methyls (ionic and neutral), ionic metal methylidenes and metal methylidynes, and bis-methyl metal ions. The periodic trends in this thermochemistry are discussed with an eye on developing ways of applying the data to condensed phase species. The measurement of gas phase BDEs of transition metal species is just emerging from its infancy. The technological and theoretical tools necessary to obtain both accurate and precise values using beam techniques have only recently been developed, as have competing methods (such as photodisso ciation 28 and proton affinity measurements 43,44). In combination with values from more established techniques (such as high temperature mass spectrometry 45 and spectroscopy), this gas phase thermochemistry provides a wealth of data which can be analysed for its periodic trends. As outlined above, these analyses can provide intrinsic bond energies for ionic and neutral metal hydrides (57–60 kcal mol −1), ionic and neutral metal methyls [2–8 kcal mol −1 higher than D 0(MH +) for cationic species, and ∼ 2 kcal mol −1 less than D 0 (MH) for neutral species], ionic metal methylidenes ( ∼ 94 kcal mol −1) and ionic metal methylidynes (∼ 130 kcal mol −1). Further, the periodic trends can help to identify influential factors in determining the strength or weakness of a bond. These include electronic (essentially hybridization) and polarization effects. Continued studies on even more complex transition metal species should enable even closer relationships between gas and condensed phase thermochemistry to be forged.