Ligation kinetics as a probe for gas-phase ligand field effects: Ligation of atomic transition metal cations with ammonia at room temperature.

The kinetics of ammonia ligation to atomic first and second row transition metal cations were measured in an attempt to assess the role of ligand field effects in gas-phase ion-molecule reaction kinetics. Measurements were performed at 295 ± 2 K in helium bath gas at 0.35 Torr using an inductively coupled plasma/selected-ion flow tube tandem mass spectrometer. The atomic cations were produced at ca. 5500 K in an inductively coupled plasma source and were allowed to decay radiatively and to thermalize by collisions with argon and helium atoms prior to reaction. A strong correlation was observed across the periodic table between the measured rate coefficients for ammonia ligation and measured/calculated bond dissociation energies. A similar strong correlation is seen with the ligand field stabilization energy. So ligand field stabilization energies should provide a useful predictor of relative rates of ligation of atomic metal ions.