Combined X-ray Diffraction and Density Functional Study of [Ni(NO)(η⁵-Cp*)] in the Ground and Light-Induced Metastable States

The crystal structure of [Ni(NO)(η⁵-Cp*)] in the light-induced metastable state was determined by X-ray diffraction at 25 K of a crystal with a 47% metastable-state population. The most significant geometrical change is the formation of a side (η²) bound structure with an Ni-N-O angle of 92(1)°, compared with 179.2(2)° in the most stable configuration, and a corresponding Ni-O distance of 2.09 Å. An elongation of the Ni-N bond by 0.08(1) Å and local distortions in the pentamethylcyclopentadienyl ring are also observed. Geometry optimizations, carried out using density functional theory, confirm that the [Ni(η²-NO)(η⁵-Cp*)] structure corresponds to a local minimum with energy 0.99 eV above that of the stable isomer and predict a second local minimum at 1.85 eV for the isonitrosyl, [Ni(ON)(η⁵-Cp*)], structure. Geometrical parameters obtained from the theoretical calculation for [Ni(η²-NO)(η⁵-Cp*)] agree reasonably well with the experimental findings. This is the first example of a side-bound nitrosyl complex generated by photoirradiation of an {M(NO)}¹⁰ ground-state configuration. Its geometry is comparable with that of the photoinduced metastable state (MS₂) of sodium nitroprusside, which in its ground state has the {M(NO)}⁶ configuration.