Low-temperature IR experiments on crystalline samples of trans-[Ru(NH3)(4)(NO) nicotinamide](3+) salts show a light-induced absorption band typical for MS 1 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 degrees 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.