Gas-phase measurements of the kinetics for the sequential ligation of Ru+ with ammonia at room temperature show an anomalous rise in the rate of addition of the fourth ammonia ligand unlike in the ammonia kinetics of Mg+, Fe+, and Ag+. The measurements were conducted with an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. B3LYP/ DZVP calculations of ligation free energies for the ammonia ligation of ground-state Ru+(F-4) and of Ru+(D-2) indicate a reversal in the relative ligation free energy that favors the ligation of Ru+(D-2) in the addition of the fourth ammonia ligand. Since the rate of ligation is dependent on the free energy of ligation, these results are consistent with the occurrence of a spin surface-crossing in the sequential ligation of Ru+(F-4) with ammonia.