Steady‐State and Pseudo‐Steady‐State Photocrystallographic Studies on Linkage Isomers of [Ni(Et 4 dien)(η 2 ‐O,ON)(η 1 ‐NO 2 )]: Identification of a New Linkage Isomer

At temperatures below 150 K, the photoactivated metastable endo ‐nitrito linkage isomer [Ni(Et 4 dien)(η 2 ‐O,ON)(η 1 ‐ONO)] (Et 4 dien= N,N,N′,N′ ‐tetraethyldiethylenetriamine) can be generated with 100 % conversion from the ground state nitro‐(η 1 ‐NO 2 ) 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‐(η 1 ‐NO 2 ) 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 E act =+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‐(η 1 ‐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 pseudo‐steady‐state methods.