Magneto-structural Correlations in Ni 2+ -Halide···Halide-Ni 2+ Chains.

We present the magnetic properties of a new family of S = 1 molecule-based magnets, NiF 2 (3,5-lut) 4 ·2H 2 O and NiX 2 (3,5-lut) 4 , where X = HF 2 , Cl, Br, or I (lut = lutidine C 7 H 9 N). Upon creation of isolated Ni-X···X-Ni and Ni-F-H-F···F-H-F-Ni chains separated by bulky and nonbridging lutidine ligands, the effect that halogen substitution has on the magnetic properties of transition-metal-ion complexes can be investigated directly and in isolation from competing processes such as Jahn-Teller distortions. We find that substitution of the larger halide ions turns on increasingly strong antiferromagnetic interactions between adjacent Ni 2+ ions via a novel through-space two-halide exchange. In this process, the X···X bond lengths in the Br and I materials are more than double the van der Waals radius of X yet can still mediate significant magnetic interactions. We also find that a simple model based on elongation/compression of the Ni 2+ octahedra cannot explain the observed single-ion anisotropy in mixed-ligand compounds. We offer an alternative that takes into account the difference in the electronegativity of axial and equatorial ligands.