Valence tautomerism in molecule-based f-block materials remains virtually elusive. As a result, the effects driving and controlling the valence conversion phenomenon are poorly understood. Herein, we unravel these fundamental factors by systematic chemical modification of a bona fide lanthanide coordination solid, SmI2(pyrazine)2(tetrahydrofuran), in which a complete, temperature-driven conversion between Sm(II) and Sm(III) occurs abruptly around 200 K. Solid solutions incorporating either divalent, diamagnetic metal ions or Sm(III) ions feature disparate behavior. Substitution with redox-inactive, divalent metal ions invariably leads to lower conversion temperatures and reduced cooperativity. In contrast, incorporation of redox-inactive Sm(III) ions leads to trapped pyrazine anion radicals in the ligand scaffold, shifting the valence tautomeric conversion phenomenon towards higher temperature with virtually no loss of cooperativity. These materials are rare examples of lanthanide–organic materials hosting mixed valency in both the lanthanide and organic scaffold, affording switchable conductivity associated with the valence tautomeric conversion.