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)(eta(5)-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 (eta(2)) bound structure with an Ni-N-O angle of 92(1)degrees, compared with 179.2(2)degrees in the most stable configuration, and a corresponding Ni-O distance of 2.09 Angstrom. An elongation of the Ni-N bond by 0.08(1) Angstrom and local distortions in the pentamethylcyclopentadienyl ring are also observed. Geometry optimizations, carried out using density functional theory, confirm that the [Ni(eta(2)-NO)(eta 5-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)(eta(5)-Cp*)], structure. Geometrical parameters obtained from the theoretical calculation for [Ni(eta(2)-NO)(eta(5)-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 (MS(2)) of sodium nitroprusside, which in its ground state has the {M(NO)}(6) configuration.