Thermal and photochemical control of nitro–nitrito linkage isomerism in single-crystals of [Ni(medpt)(NO 2)(η 2 -ONO)] ac The known complex [Ni(medpt)(η 1 -NO 2)(η 2 -ONO)] 1 (medpt = 3,3′-diamino-N-methyldipropylamine) crystallises in the monoclinic space group P2 1 /m with 1.5 molecules in the asymmetric unit with two different η 1 -NO 2 ligand environments in the crystal structure. At 298 K the molecule (A) sitting in a general crystallographic site displays a mixture of isomers, 78% of the η 1 -NO 2 isomer and 22% of an endo-nitrito–(η 1 -ONO) form. The molecule (B) sitting on a crystallographic mirror plane adopts the η 1 -NO 2 isomeric form exclusively. However, a variable temperature crystallographic study showed that the two isomers were in equilibrium and upon cooling to 150 K the η 1 -ONO isomer converted completely to the η 1 -NO 2 isomer, so that both independent molecules in the asymmetric unit were 100% in the η 1 -NO 2 form. A kinetic analysis of the equilibrium afforded values of ΔH = −9.6 (± 0.4) kJ mol −1 , ΔS = −21.5 (±1.8) J K −1 mol −1 and E A = −1.6 (±0.05) kJ mol −1 . Photoirradiation of single crystals of 1 with 400 nm light, at 100 K, resulted in partial isomerisation of the η 1 -NO 2 isomer to the metastable η 1 -ONO isomer, with 89% for molecule (A), and 32% for molecule (B). The crystallographic space group also reduced in symmetry to P2 1 with Z′ = 3. The metastable state existed up to a temperature of 150 K above which temperature it reverted to the ground state. An analysis of the crystal packing in the ground and metastable states suggests that hydrogen bonding is responsible for the difference in the conversion between molecules (A) and (B).