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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.