A Tetrameric Nickel( ) TMChair∫ with both Antiferromagnetic Internal Coupling and Ferromagnetic Spin Alignment**

Interest in polynuclear complexes of the 3d metals has been stimulated by the search for new magnetic materials[1, 2] and by demonstration of the occurrence of oligonuclear metal centers in proteins such as urease.[3] Of the relatively small number of reported tetranuclear complexes of S 1 nickel( ) of known structure, the majority have a hemicubaneor TMbutterfly-∫ rather than a squarelike core.[4] Of these molecules, just one is entirely antiferromagnetic,[5] while the remainder entail purely ferromagnetic interactions amongst the nickel( ) ions.[6±10] Oximes have shown promise as bridging ligands for the preparation of polynuclear complexes.[11, 12] The reaction of 1,4,7-triazaheptane (diethylenetriamine, Dien) with the monooxime of 2,3-butanedione (ModaH) in the presence of NiII ions, instead of yielding the anticipated Schiff base derivative, gave the tetranuclear NiII compound 1 (dark brown crystals; C4H8O2 1,4-dioxane), containing uncondensed but coordinated ketone and amine groups. Figure 1a shows the structure of the the cation of 1, while Figure 1b highlights its NiII core.