Inertness of a Planar Ni 7 O 6 + Cluster in CO Adsorption Revealed by Ion Mobility–Mass Spectrometry

A previous mass spectrometry study reported that the CO adsorption rate constant for the Ni 7 O 6 + 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 Ni 7 O 6 + : three‐dimensional (3D) and two‐dimensional (2D) structures. The CO adsorption rate constant for the 2D isomer of Ni 7 O 6 + was determined to be <6 × 10 −12 cm 3 molecule −1 s −1 , which is significantly smaller than that of the 3D isomer (1.3 × 10 −10 cm 3 molecule −1 s −1 ). Molecular orbital analysis reveals that the π‐back donation cannot occur between the 2D isomer of Ni 7 O 6 + and a CO molecule due to the absence of d yz orbital character on the Ni atoms. We conclude that the extraordinary stability of Ni 7 O 6 + originates directly from the character of its constituent Ni atomic orbitals in the 2D isomer.