Thermally induced phase transition and X-ray crystal structure of bis(N,N′-dimethyl-1,2-ethanediamine)di-isothiocyanatonickel(II)

The complex [Ni(MeNHCH₂CH₂NHMe)₂(NCS)₂] (1) has been synthesised and characterised by X-ray crystallography. It undergoes an irreversible endothermic phase transition (452.5-468.5 K, ΔH = 13.4 kJ mol^-1) to species (2) which reverts to (1) in a humid atmosphere. Complex (1) crystallises in the monoclinic space group C2/c with a = 13.069(2), b = 9.783(3), c = 15.074(4) Å, β = 111.40(2)°, and Z = 4. The structure has been solved by Patterson and successive Fourier methods and refined to R = 0.032 for 1 240 independent observed reflections. The co-ordination around the nickel atom, lying on a crystallographic inversion centre, is close to octahedral with the two thiocyanate N atoms in trans position. The chelate rings exist in the δλ conformation. For complex (2), a = b = 8.16(3), c = 7.53(2) Å, α = β = 107.0(2), γ = 73.6(3)°, and Z = 1. Crystallographic and i.r. spectroscopic data suggest a non-centrosymmetric structure for (2) with a qualitative change in the conformation of the complex to δδ or λλ.