COMBINED X-RAY-DIFFRACTION AND DENSITY-FUNCTIONAL STUDY OF [NI(NO)(ETA(5)-CP-ASTERISK)] IN THE GROUND AND LIGHT-INDUCED METASTABLE STATES

The crystal structure of [Ni(NO)(eta(5)-Cp)] in the light-induced met astable state was determined by X-ray diffraction at 25 K of a crystal with a 47% metastable-state population. The most significant geometri cal 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 mo st stable configuration, and a corresponding Ni-O distance of 2.09 Ang strom. An elongation of the Ni-N bond by 0.08(1) Angstrom and local di stortions in the pentamethylcyclopentadienyl ring are also observed. G eometry optimizations, carried out using density functional theory, co nfirm that the [Ni(eta(2)-NO)(eta 5-Cp)] structure corresponds to a l ocal minimum with energy 0.99 eV above that of the stable isomer and p redict a second local minimum at 1.85 eV for the isonitrosyl, [Ni(ON)( eta(5)-Cp)], structure. Geometrical parameters obtained from the theo retical calculation for [Ni(eta(2)-NO)(eta(5)-Cp)] agree reasonably w ell with the experimental findings. This is the first example of a sid e-bound nitrosyl complex generated by photoirradiation of an {M(NO)}(1 0) ground-state configuration. Its geometry is comparable with that of the photoinduced metastable state (MS2) of sodium nitroprusside, whic h in its ground state has the {M(NO)}(6) configuration.