The general rule that in Ni(II) d(8) chemistry, tetrahedral (or nearly tetrahedral) complexes have temperature dependent magnetic moments which are usually larger than the spin-only value whilst square planar complexes are diamagnetic is broken for certain Ni[P((t)Bu)(2)(O)NR](2) complexes. These have planar coordination for various alkyl groups but have the spectral and paramagnetic properties normally associated with tetrahedral systems. In contract to previous studies, density functional calculations show the unusual adoption of a high-spin rather than low-spin arrangement in the planar systems is due to the strong pi bonding of the amide group and the preference for a planar coordination is due to greater separation between the bulky nitrogen and phosphorus substituents.