Exceptional Manganese(II) Stability and Manganese(II)/Zinc(II) Selectivity with Rigid Polydentate Ligands**

While MnII complexes meet increasing interest in biomedical applications, ligands are lacking that enable high complex stability and selectivity vs. ZnII, the most relevant biological competitor. We report here two new bispidine derivatives, which provide rigid large coordination cavities perfectly match size of MnII, yielding eight-coordinate with record stabilities. In contrast, smaller ZnII ion cannot accommodate all ligand donors, resulting highly strained less stable six-coordinate complexes. Combined theoretical experimental data (X-ray crystallography, potentiometry, relaxometry 1H NMR spectroscopy) demonstrate unprecedented for (KMnL/KZnL 108–1010), sharp contrast to usual Irving–Williams behavior, stabilities inertness logKMnL close 25.