Heterobimetallic Mn/Fe cofactors are found in the R2 subunit of class Ic ribonucleotide reductases (R2c) and R2-like ligand binding oxidases (R2lox). Selective cofactor assembly is due at least in part to the thermodynamics of M-II binding to the apoprotein. We report here equilibrium studies of Fe-II/Mn-II discrimination in the biomimetic model system H-5(F-HXTA) (5-fluoro-2-hydroxy-1,3-xylen