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

The brown nitro isomer, trans-[NiL2(NO2)(2)], 1 (L=N,N'-dipropyl-1,2-diamin oethane), was synthesized by adding L to a suspension of K-4[Ni(NO2)(6)].H2 O in methanol. Complex 1 is metastable at ca. 298 K and transforms slowly t o the greenish blue nitrito(O,O) isomer, cis-[NiL2(O2N)](NO2), 2. This tran sformation is accelerated with increasing temperature and slowed down on lo wering the temperature (< 283 K). Complex 2 shows a reversible phase transi tion, 2 <---->3 (cooling: DeltaH=8.150 +/-0.004 kJ mol(-1); 266-255 K; heat ing: DeltaH=-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 cha racterized 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 transform ation is supposed to be due to the change in thermal motion of the side cha ins on variation of temperature.