Photo-induced linkage isomerism of transition metal nitrosyl and dinitrogen complexes studied by photocrystallographic techniques

New metastable species are formed by low-temperature irradiation of crystal s of transition metal nitrosyl complexes. In the work described, photocryst allographic techniques, in which crystals are exposed in situ, are combined with differential scanning calorimetry measurements, IR spectroscopy and t heoretical calculations. The new species are identified as isonitrosyl and eta(2)-NO linkage isomers, which revert to the ground state on subsequent w arming. The stability of the linkage isomers increases with increasing pi-w ithdrawing ability of the ligand trans to NO, indicating the crucial role o f electronic factors in determining relative stability of the isomers of di fferent complexes. Isomers with decay temperatures up to similar to 0 degre es C 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 o bservation of different photochemical behavior when [RU(NO)(NH3)(5)](NO3)(3 ) is absorbed in a sol-gel glass. The two independent molecules in the asym metric unit of [Ni(NO)(eta(5)-Cp*)] show different orientations of the eta( 2)-NO ligand after irradiation, the difference bring related to the shape o f the reaction cavities at the two sites. A previously unknown side-on bind ing mode, similar to that occurring for NO, has been discovered for the din itrogen ligand in [Os(NH3)(5)(N-2)][PF6](2), indicating the potential of th e techniques used in the search for new species and novel binding modes of transition metal atoms. (C) 2000 Elsevier Science Ltd. All rights reserved.