Photo-Induced Linkage Isomerism of Transition Metal Nitrosyl and Dinitrogen Complexes Studied by Photocrystallographic Techniques Dmitry V. Fomitchev, Irina Novozhilova and Philip Coppens * Department of Chemistry, Natural Sciences Complex, State University of New York at Buffalo, Buffalo, NY 14260-3000, USA Received 29 December 1999; revised 7 March 2000; accepted 10 March 2000 Abstract ÐNew metastable species are formed by low-temperature irradiation of crystals of transition metal nitrosyl complexes. In the work described, photocrystallographic techniques, in which crystals are exposed in situ, are combined with differential scanning calorimetry measurements, IR spectroscopy and theoretical calculations. The new species are identi®ed as isonitrosyl and h 2 -NO linkage isomers, which revert to the ground state on subsequent warming. The stability of the linkage isomers increases with increasing p-withdrawing ability of the ligand trans to NO, indicating the crucial role of electronic factors in determining relative stability of the isomers of different complexes. Isomers with decay temperatures up to ,08C have been prepared. Solid state effects are evident from comparison of the behavior of different salts of the same ion, of sodiumnitroprusside in the neat crystal and in a co-crystal with 18-crown-6 ether, and from the observation of different photochemical behavior when [Ru(NO)(NH 3 ) 5 ](NO 3 ) 3 is absorbed in a sol±gel glass. The two independent molecules in the asymmetric unit of [Ni(NO)(h 5 -Cp p )] show different orientations of the h 2 -NO ligand after irradiation, the difference being related to the shape of the reaction cavities at the two sites. A previously unknown side-on binding mode, similar to that occurring for NO, has been discovered for the dinitrogen ligand in [Os(NH 3 ) 5 (N 2 )][PF 6 ] 2 , indicating the potential of the techniques used in the search for new species and novel binding modes of transition metal atoms. q 2000 Elsevier Science Ltd. All rights reserved. Introduction The effect of light on crystals of sodium nitroprusside dihydrate (Na 2 [Fe(CN) 5 (NO)]´2H 2 O, or SNP, was dis- covered in 1977. 1 At that time and long after, the light- induced species were classi®ed as electronically excited states, even though their unusually long lifetimes at low temperature raised questions about this interpretation, given the diamagnetic nature of the light-induced species. A large body of valuable information on the thermo- dynamics, kinetics and other properties of SNP and other nitroprusside salts was obtained in subsequent studies (see for example Refs. 2±5). In the same year as the discovery of the light-induced meta- stable states of SNP, but apparently quite unrelated, Rest and co-workers observed light-induced changes in [Ni(NO)(h 5 -Cp)], 6 and concluded that electron transfer from the metal to the nitrosyl ligand was involved, with possible bending of the M±N±O group. Perhaps because SNP is an {MNO} 6 complex in terms of the notation intro- duced by Enemark and Feltham, 7 while Ni(NO)Cp is {MNO}, 10 the relation between the two sets of observations appears not to have been noticed for at least a decade. Our initial interest in the light-induced transition metal nitrosyl species stemmed from their classi®cation as elec- tronically excited states. Given the long lifetimes (t .10 7 s at liquid nitrogen temperature), the compounds seemed intriguing candidates for excited state diffraction studies. In the synchrotron-based experiments on excited state species, low-temperature diffraction is combined with in situ laser-excitation, and pulsed laser and X-ray sources are used in time resolved experiments of short-lived species. We have proposed the name photocrystallography to describe the ®eld, which combines photochemistry and crystallography in single experiments. The work described here did not require synchrotron radiation, but provided valuable experience in combining the crystallographic and spectroscopic techniques. Techniques Used In situ irradiation of a diffractometer-mounted, cryogeni- cally cooled crystal is the preferred method for studying the geometry of geometry short-lived metastable and excited species. In our ®rst experiments this was accom- plished with liquid nitrogen cooling and a classical four- circle diffractometer equipped with a scintillation detector. Tetrahedron 56 (2000) 6813±6820 Pergamon TETRAHEDRON 0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0040-4020(00)00503-2 Keywords: photocrystallography; nitrosyl compounds; side-bound dinitro- gen; reaction cavity; solid state effects; isonitrosyl complexes; side-bound nitrosyl; DSC; infrared spectroscopy; X-ray analysis. * Corresponding author. Fax: 11-716-645-6948; e-mail: coppens@acsu. buffalo.edu