Steady- State and Pseudo-Steady-State Photocrystallographic Studies on Linkage Isomers of [Ni( Et4dien)(n2-O,ON)(n1-NO2)]: Identification of a New Linkage Isomer

At temperatures below 150K, the photoactivated metastable endo-nitrito linkage isomer [Ni(Et(4)dien)((2)-O,ON)((1)-ONO)] (Et(4)dien=N,N,N,N-tetraethyldiethylenetriamine) can be generated with 100% conversion from the ground state nitro-((1)-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-((1)-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 E-act=+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-((1)-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.