Infrared spectra of mass-selected homoleptic nickel carbonyl cluster cations including dinuclear Ni2(CO)_7~+ and Ni2(CO)_8~+, trinuclear Ni3(CO)_9~+ and tetranuclear Ni4(CO)_(11)~+ are measured via infrared) photodissociation spectroscopy in the carbonyl stretching frequency region. The structures are established by comparison of the experimental spectra with simulated spectra derived from density functional calculations. The Ni2(CO)_7~+ cation is characterized to have an unbridged asymmetric (OC)4Ni-Ni(CO)_3~+ structure with a Ni-Ni single bond. The Ni2(CO)_8~+ cation has a Ni-Ni half-bonded D_(3d) structure) with both nickel centers exhibiting an 18-electron configuration. The trinuclear Ni3(CO)_9~+ cluster cation is determined to have an open chain like (0C)4Ni-NiCO-Ni(CO)4 structure. The tetranuclear Ni4(CO)_(11)~+ cluster cation is determined to have a tetrahedral structure with two-center and three-center bridge-bonded carbonyl units. These nickel carbonyl cluster cations all involve trigonal pyramid like Ni(CO)4 building blocks that satisfy the 18-electron configuration of the nickel centers.
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