On the Photochemical Behavior of the [Ru(NH₃)₄(NO)nicotinamide]³⁺ Cation and the Relative Stability of Light-Induced Metastable Isonitrosyl Isomers of Ru Complexes

Low-temperature IR experiments on crystalline samples of trans-[Ru(NH3)4(NO) nicotinamide]3+ salts show a light-induced absorption band typical for MS1 NO linkage isomers upon exposure to 300−500 nm light from a Xe source. The formation of a metastable species is confirmed by DSC measurement on a sample irradiated at low temperature with 457 nm light from an Ar+ laser. The light-induced species decays between 250 and 260 K according to both IR and DSC results. This decay temperature (Td) is somewhat below that observed for other high-Td linkage isomers, even though the NO-stretching frequency of the of [Ru(NH3)4(NO) nicotinamide]3+ ion is above that of the other isomers, demonstrating a lack of precise correlation between the two physical properties. The 90 K crystal structure of trans-[Ru(NH3)4(NO)nicotinamide](SiF6)(NO3)·H2O is reported. The geometry from theoretical DFT calculations of the ground-state structure agrees well with the experimental results, except for the orientation of the CONH2 substituent in the pyridine ring, which is rotated by 180° in the crystal due to packing effects. The MS1 and MS2 linkage isomers are found to correspond to local minima on the ground-state potential energy surface, and their geometries and energies are reported.