Keywords: Transfer hydrogenation / Ruthenium / C-H activation / Mass spectrometry / Density functional calculationsRuthenium(II) complexes [(η 6 -cymene)RuCl(apypm)]BPh 4 with bidentate 2-amino-4-(2-pyridinyl)pyrimidine (apypm) ligands catalyze the transfer hydrogenation of acetophenone. Their activities are strongly dependent on the substituent pattern of the pyrimidine ring. Complexes bearing a primary amino group in the 2-position of the pyrimidine ring do not perform the catalysis in terms of a "bifunctional mechanism", although they possess protic hydrogen atoms at the amino moiety in close proximity to the metal site. Systems contain-

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The synthesis of 2‐substituted pyridine–pyrimidine ligands and their complexation with arene ruthenium(II) chloride moieties is reported. Depending on the electronic and steric influences of the ligand, the catalysts undergo CH activation by roll‐over cyclometalation. This process opens up the route to the catalytic transfer hydrogenation of ketones with isopropanol as the hydrogen source under base‐free and mild conditions. Barriers related to the roll‐over cyclometalation process can be determined experimentally by collision‐induced dissociation ESI mass spectrometry. They are supported by DFT calculations and allow the classification of the ligands according to their electronic and steric properties, which is also in accordance with critical bond parameters derived from X‐ray structure data. DFT calculations furthermore reveal that the formation of a ruthenium(II) hydrido species is plausible through β‐hydride elimination from isopropanol.

We have carried out experimental and theoretical studies of Penning ionization processes occurring in thermal energy collisions of state-selected metastable He*(2S) and He*(2S) atoms with ground state alkaline earth atoms X(X"Mg, Ca, Sr, Ba). Penning ionization electron energy spectra for these eight systems, measured with a crossed-beam set-up perpendicular to the collision velocity at energy resolutions 40-70 meV, are reported; relative populations of the different ionic X> (ml) states are presented and well depths D* C for the He*#X entrance channel potentials with uncertainties around 25 meV are derived from the electron spectra as follows: He*(2S)#Mg/Ca/Sr/Ba: 130/250/240/260 meV; He*(2S) #Mg/Ca/Sr/Ba: 300/570/550/670 meV. The spectra show substantial differences for the three ionic states X>(S), X>(P) and X>(D) and reveal that transitions to a repulsive potential -attributed to He#X>(P) formation -are mainly involved for the X>(P) channel. Ab initio calculations of potential curves, autoionization widths, electron energy spectra and ionization cross sections are reported for the systems He*(2S)#Ca and He*(2S)#Ca. The respective well depths D* C are calculated to be 243(15) meV and 544 (15) meV; the ionization cross sections at the experimental mean energy of 72 meV amount to 101 A s and 201 A s , respectively. Very good overall agreement with the experimental electron spectra is observed.