At temperatures below 150K, the photoactivated metastable endo-nitrito linkage isomer [Ni(Et4dien)(η2-O,ON)(η1-ONO)] (Et4dien=N,N,N',N'-tetraethyldiethylenetriamine) can be generated with 100% conversion from the ground state nitro-(η1-NO2) isomer on irradiation with 500nm light, in the single crystal by steady-state photocrystallographic techniques. Kinetic studies show the system is no longer metastable above 150K, decaying back to the ground state nitro-(η1-NO2) arrangement over several hours at 150K. Variable-temperature kinetic measurements in the range of 150-160K show that the rate of endo-nitrito decay is highly dependent on temperature, and an activation energy of Eact=+48.6(4)kJmol-1 is calculated for the decay process. Pseudo-steady-state experiments, where the crystal is continually pumped by the light source for the duration of the X-ray experiment, show the production of a previously unobserved, exo-nitrito-(η1-ONO) linkage isomer only at temperatures close to the metastable limit (ca. 140-190K). This exo isomer is considered to be a transient excited-state species, as it is only observed in data collected by pseudo-steady-state methods.