IR and theoretical studies of monocarbonyl Ni complexes formed by adsorption of CO at low pressure on silica-supported NiII ions

This work reports on the reactivity of coordination vacancies of Ni-II ions grafted onto the tridentate silica support (Ni-3c(II) ions) with respect to CO used as a probe molecule. The adsorption of CO at 77 K in the 0.3 to 3.5 Pa CO pressure range is studied by FTIR on two samples differing in the dispersion of nickel. Quantum chemical calculations by the DFT method are performed to investigate, using a cluster approach, the binding of Ni to silica and, after CO adsorption, the geometry of the resulting carbonyl Ni complexes. Silica is modeled by using clusters composed of three types of monodentate ligands, SiO-, SiOSi and/or SiOH, found on the surface of silica. This work is devoted to the monocarbonyl complexes, Whatever the sample, only one type of monocarbonyl is formed from Ni-3c(II) ions. It is shown that the charge of the silica cluster is the major parameter influencing the CO IR frequency whereas the nature and the size of the silica cluster do not affect the CO bond length, confirming that local electrostatic interactions predominate. Only the 1 - charged silica cluster Si5O3-, composed of SiO-, 2SiOSi fragments, respectively, reproduces the Ni-O distances derived from EXAFS for the Ni-3c(II) grafted site and gives CO frequencies in good agreement with the experimental values. It is shown that CO is stabilized by a magnetic transition from the Ni-3(2+) triplet to the Ni-1(2+) singlet state occurring upon adsorption.