Inertness of a Planar Ni7O6 + Cluster in CO Adsorption Revealed by Ion Mobility-Mass Spectrometry.

A previous mass spectrometry study reported that the CO adsorption rate constant for the Ni7O6 + cluster is anomalously smaller than those of adjacent sizes. In this study, the origin of this remarkable inertness was revealed using a combination of ion mobility-mass spectrometry (IM-MS) and theoretical calculations. IM-MS measurements clearly resolved two distinct structural isomers of Ni7O6 +: three-dimensional (3D) and two-dimensional (2D) structures. The CO adsorption rate constant for the 2D isomer of Ni7O6 + was determined to be <6 × 10-12 cm3 molecule-1 s-1, which is significantly smaller than that of the 3D isomer (1.3 × 10-10 cm3 molecule-1 s-1). Molecular orbital analysis reveals that the π-back donation cannot occur between the 2D isomer of Ni7O6 + and a CO molecule due to the absence of dyz orbital character on the Ni atoms. We conclude that the extraordinary stability of Ni7O6 + originates directly from the character of its constituent Ni atomic orbitals in the 2D isomer.