Ni and CO used as probes of the amorphous silica surface:: IR and theoretical studies of dicarbonyl NiII complexes

This paper is the continuation of a study of Ni-II-(CO)(n) complexes formed on amorphous silica. Both Ni and CO are used as successive probes of the local structure and reactivity of the silica surface. An isolated tricoordinated Ni-3c(II) ion in a single oxidation state is characterized with CO as a probe of the coordination vacancies of the transition metal. An experimental FTIR study shows two main types of Ni-II dicarbonyl with equivalent and nonequivalent COs formed from the Ni-II monocarbonyl previously characterized in J. Am. Chem. Soc. 2002, 24, 7210-7217, whereas no tricarbonyl Ni-II complex is observed. Theoretical models are tested using experimental CO frequencies: DFT calculations with the cluster approach already used for the Ni-II monocarbonyl complex allow us to gain valuable information on the local environment of the Ni-II ion. For Ni-II dicarbonyls, information can be obtained on the second coordination sphere of Ni-II (namely, Si atoms) whereas for monocarbonyls, only first oxygen neighbors were concerned. Then, by embedding the small silica fragments in silica rings really existing on the surface, we can bridge the reality gap between the theoretical model and its application to IR studies. We show that the Ni-3c(II) ion is stabilized by an 8T or 9T ring with an adjacent strained 2T ring (where nT represents the number of silicon atoms in a ring).