At ambient temperature, the complex [Ni(OAc)(PNPtBu)]OTf has one independent molecule in the asymmetric unit of the crystal structure with very large anisotropic displacement parameters. During cooling a fully reversible solid-solid phase transition occurs at a discrete temperature in the range 210-230 K. The low-temperature phase has six independent, well ordered molecules in the asymmetric unit. The P2(1)/a space group symmetry of the high-temperature phase changes to P2(1)/n for the low-temperature phase and the c-axis increases by a factor of six. The acetate ligand is shown to be coordinated in a eta(1)-fashion through one of the oxygen atoms, with the sterically encumbered, tridentate PNPtBu ligand completing the square planar geometry around the Ni-II ion. The synthesis and full characterization of the complex is reported. (C) 2009 Elsevier Ltd. All rights reserved.