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 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 identified as isonitrosyl and η2-NO linkage isomers, which revert to the ground state on subsequent warming. The stability of the linkage isomers increases with increasing π-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 ∼0°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 observation of different photochemical behavior when [Ru(NO)(NH3)5](NO3)3 is absorbed in a sol–gel glass. The two independent molecules in the asymmetric unit of [Ni(NO)(η5-Cp∗)] show different orientations of the η2-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(NH3)5(N2)][PF6]2, indicating the potential of the techniques used in the search for new species and novel binding modes of transition metal atoms.