Ionic liquids (ILs) derive their useful properties from molecularly tunable compositions, but methods to diversify anion identities and probe ion speciation remain limited. Here, we demonstrate post-synthetic modification of perhalometallate anions to achieve ionic liquid-to-ionic liquid transformations. Rheological measurements of the metal-containing ILs indicate that minor alterations to anion coordination spheres induces considerable changes to IL viscosities. UV-vis spectra confirm the purities for most ILs, while revealing a surprising cation dependence of perchlorovanadate speciation and supramolecular structure. The intermolecular interactions studied here span a wide range from dispersion to covalent bonding, permitting their impact on IL viscosity to be decoupled and quantified. Together, synthetic strategies from coordination chemistry paired with conventional UV-vis spectroscopy provide a powerful tool for expanding IL compositions and investigating fundamental nanoscale behavior.