STRONG HYDROGEN-BONDING IN GAS-PHASE IONS - A HIGH-PRESSURE MASS-SPECTROMETRIC STUDY OF THE PROTON AFFINITY, PROTON-TRANSFER KINETICS, AND HYDROGEN-BONDING CAPABILITY OF IRON PENTACARBONYL

Pulsed-ionization high-pressure mass spectrometric investigations of the proton transfer and clustering reactions of protonated iron pentacarbonyl have been carried out. The slow proton transfer kinetics and negative temperature dependence of the reaction with mesitylene are consistent with a barrier to proton transfer within the intermediate ion-molecule complex. Arguments are presented that show that the loss of the fluxional motion in Fe(CO)5 upon protonation leads to an unusually large entropy change. The proton affinity of Fe(CO)5 is determined to be 200.3 kcal mol-1. The clustering of protonated iron pentacarbonyl with acetonitrile is found to be relatively strong at 17 kcal mol-1 and is indicative of a hydrogen-bonded complex involving the transition-metal center. Collision-induced decomposition experiments support metal protonation of Fe(CO)5 but ring protonation of ferrocene in the gas phase. This difference in site of protonation is invoked to explain the differences in hydrogen bond strengths in the proton-bound complexes of the two transition-metal species with acetonitrile.