Identification of a New Linkage Isomer**

At temperatures below 150 K, the photoactivated metastable endo-nitrito linkage isomer [Ni(Et4dien)(h O,ON)(h-ONO)] (Et4dien = N,N,N’,N’-tetraethyldiethylenetriamine) can be generated with 100 % conversion from the ground state nitro-(h-NO2) isomer on irradiation with 500 nm light, in the single crystal by steady-state photocrystallographic techniques. Kinetic studies show the system is no longer metastable above 150 K, decaying back to the ground state nitro-(h-NO2) arrangement over several hours at 150 K. Variable-temperature kinetic measurements in the range of 150–160 K show that the rate of endo-nitrito decay is highly dependent on temperature, and an activation energy of Eact = + 48.6(4) kJ mol 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-(h-ONO) linkage isomer only at temperatures close to the metastable limit (ca. 140–190 K). This exo isomer is considered to be a transient excited-state species, as it is only observed in data collected by pseudosteady-state methods.