Coordination of triply charged lanthanum in the gas phase: theory and experiment.

Ion-molecule reactions between complexes [La(CH3CN)n]3+ (n=6-9) or [La(NC(CH2)4CN)n]3+ (n=3-4) and water were studied at low collision energies in the second quadrupole of a tandem mass spectrometer. The products [La(CH3CN)p(H2O)8-p]3+ (p=6-8) and [La(NC(CH2)4CN)q(H2O)8-2q]3+ (q=3-4) had the highest relative abundances. This strongly suggests that the preferred coordination number of La3+ is eight. Similarly, the coordination number of Ca2+ was re-examined both experimentally and theoretically, and was found to be six, in good agreement with previous observations. Density functional calculations provide strong evidence that the primary solvation shell of [La(L)n]3+ consists of eight ligands; additional ligands reside in a second solvation shell and are hydrogen bonded to one or two water molecules in the first shell.