The First Side-bound NiNO Complex: X-ray Diffraction Study of the Ground and Excited States of [NiCp^*(NO)]. Dmitry V. Fomitchev and Philip Coppens, Chemistry Department, State University of New York at Buffalo, Buffalo, NY 14260-3000.

The geometry of the photogenerated metastable state of ([eta]⁵-pentamethylcyclopentadienyl)nitrosylnickel, [NiCp*(NO)] has been determined by low-temperature X-ray diffraction. The metastable state was generated in situ by irradiation of a cryostat-mounted crystal at 25K, with 458nm light from an Ar⁺ laser. Two data sets were collected on the same crystal before and after irradiation, the first for the ground state and the second for analysis of the mixed ground/metastable state. Least-squares refinement of the data shows the population of the metastable state to be 47%. Major geometrical changes upon excitation are almost identical for the two independent molecules, though a disorder of the nitrosyl group is found for the B, but not the A molecule. The changes are an elongation of the Ni-N bond by 0.10 Å, a dramatic bending of the Ni-N-O angle from 180deg. to 92deg. leading to a sideways-bound structure, and an increase of the Ni-Cp^* distance by 0.02 Å.

The results on the geometry of the metastable state of [NiCp*(NO)] complement our recent findings of a side-bound NO group in the second metastable state, MS₂, of sodium nitroprusside, Na₂[Fe(CN) ₅ (NO)]2H₂O [1], and of isonitrosyl configurations in the first metastable states, MS₁, of both Na₂ [Fe₂(CN) ₅ (NO)]2H₂O and K₂ [Ru(NO₂) ₄ (NO)(OH)] [2]. The results agree with those from an earlier EXAFS study of the excited state geometry of [NiCp(NO)] in solid methylpentane on the lengthening of the Ni-N bond, but not on the Ni-N-O angle, which was reported as between 133 and 160[ring] in the earlier study [3].

[1] M.D. Carducci, M.R. Pressprich and P. Coppens, J.Am.Chem.Soc. In Press. [2] D.V. Fomitchev and P. Coppens, Inorg.Chem. 35, 7021-7026 (1996). [3] L. X. Chen, M. K. Bowman, Z. Wang, P. A. Montano and J. R. Norris, J. Phys. Chem. 98, 9457-9464 (1994).

Research supported by the National Science Foundation (CHE9317770 and CHE9615586) and the Donors of The Petroleum Research Fund (PRF28664-AC3).