First report on thermally induced nitro→nitrito(O,O) linkage isomerization in diamine complexes of nickel(II) in the solid state: X-ray single crystal structural analyses of nitro and nitrito isomers

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The brown nitro isomer, trans-[NiL2(NO2)2], 1 (L = N,N′-dipropyl-1,2-diaminoethane), was synthesized by adding L to a suspension of K4[Ni(NO2)6]·H2O in methanol. Complex 1 is metastable at ca. 298 K and transforms slowly to the greenish blue nitrito(O,O) isomer, cis-[NiL2(O2N)](NO2), 2. This transformation is accelerated with increasing temperature and slowed down on lowering the temperature (< 283 K). Complex 2 shows a reversible phase transition, 2↔3 (cooling: ΔH = 8.150 ± 0.004 kJ mol−1; 266–255 K; heating: ΔH = −7.960 ± 0.003 kJ mol−1; 257–268 K), whereas complex 1 does not exhibit any such transition. The structures of 1, 2 and 3 have been characterized by X-ray single crystal structure analyses, which reveal that 1 and 2/3 are nitro and nitrito(O,O) linkage isomers, respectively. Complexes 2 and 3 have essentially similar structures and their reversible transformation is supposed to be due to the change in thermal motion of the side chains on variation of temperature.