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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.

The [Zn 1- x Ni x (HF 2 )(pyz) 2 ]SbF 6 ( x = 0.2; pyz = pyrazine) solid solution exhibits a zero-field splitting ( D ) that is 22% larger [ D = 16.2(2) K (11.3(2) cm -1 )] than that observed in the x = 1 material [ D = 13.3(1) K (9.2(1) cm -1 )]. The substantial change in D is accomplished by an anisotropic lattice expansion in the MN 4 (M = Zn or Ni) plane, wherein the increased concentration of isotropic Zn(II) ions induces a nonlinear variation in M-F and M-N bond lengths. In this, we exploit the relative donor atom hardness, where M-F and M-N form strong ionic and weak coordinate covalent bonds, respectively, the latter being more sensitive to substitution of Ni by the slightly larger Zn(II) ion. In this way, we are able to tune the single-ion anisotropy of a magnetic lattice site by Zn-substitution on nearby sites. This effect has possible applications in the field of single-ion magnets and the design of other molecule-based magnetic systems.