Solid- State Interconversions: Unique 100% Reversible Transformations between the Ground and Metastable States in Single- Crystals of a Series of Nickel( II) Nitro Complexes

The solid-state, low-temperature linkage isomerism in a series of five square planar group10 phosphino nitro complexes have been investigated by a combination of photocrystallographic experiments, Raman spectroscopy and computer modelling. The factors influencing the reversible solid-state interconversion between the nitro and nitrito structural isomers have also been investigated, providing insight into the dynamics of this process. The cis-[Ni(dcpe)(NO2)(2)] (1) and cis-[Ni(dppe)(NO2)(2)] (2) complexes show reversible 100% interconversion between the (1)-NO2 nitro isomer and the (1)-ONO nitrito form when single-crystals are irradiated with 400nm light at 100K. Variable temperature photocrystallographic studies for these complexes established that the metastable nitrito isomer reverted to the ground-state nitro isomer at temperatures above 180K. By comparison, the related trans complex [Ni(PCy3)(2)(NO2)(2)] (3) showed 82% conversion under the same experimental conditions at 100K. The level of conversion to the metastable nitrito isomers is further reduced when the nickel centre is replaced by palladium or platinum. Prolonged irradiation of the trans-[Pd(PCy3)(2)(NO2)(2)] (4) and trans-[Pt(PCy3)(2)(NO2)(2)] (5) with 400nm light gives reversible conversions of 44 and 27%, respectively, consistent with the slower kinetics associated with the heavier members of group10. The mechanism of the interconversion has been investigated by theoretical calculations based on the model complex [Ni(dmpe)Cl(NO2)].