Magneto-structural Correlations in Ni2+-Halide center dot center dot center dot Halide-Ni2+ Chains

We present the magnetic properties of a new family of S = 1 molecule-based magnets, NiF2(3,5-lut)(4)center dot 2H(2)O and NiX2(3,5-lut)(4), where X = HF2, Cl, Br, or I (lut = lutidine C7H9N). Upon creation of isolated Ni-X center dot center dot center dot X-Ni and Ni-F-H-F center dot center dot center dot 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 Ni2+ ions via a novel through-space two-halide exchange. In this process, the X center dot center dot center dot 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 Ni2+ 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.