The reaction of the segmental compound 2-[6-(diethylcarbamoyl)pyridin-2-yl]-1,1′-dimethyl-2′-(5- methylpyridin-2-yl)-5,5′-methylenebis(1H-benzimidazole) (L) with a stoichiometric mixture of Fe ^II and Ln ^III (Ln = La, Nd, Eu, Gd, Tb, Yb, Lu, Y or Sc) or Ca ^II in acetonitrile produced selectively the heterodinuclear non-covalent podates [LnFeL ₃ ] ⁵⁺ and [CaFeL ₃ ] ⁴⁺ . Proton NMR and electronic spectroscopy and electrochemistry showed that the ligands are helically wrapped around the metal ions leading to a C ₃ -triple-helical structure with Fe ^II occupying the pseudo-octahedral co-ordination site produced by the three bidentate binding units and Ln ^III lying in the remaining pseudo-tricapped trigonal-prismatic site defined by the three tridentate binding units. In this chemical environment Fe ^II sustains a thermally induced low-spin → high-spin transition around room temperature in acetonitrile, the thermodynamic parameters of which can be finely controlled by the size of the co-ordinated Ln ^III . Thermodynamic investigations of the assembly process suggest that the stability of the final complexes [LnFeL ₃ ] ⁵⁺ depends on the size of Ln ^III , small metal ions leading to intricate mixtures of complexes. The crystal structure of [LaFeL ₃ ][ClO ₄ ] _0.5 [CF ₃ SO ₃ ] _4.5 ·MeCN·4H ₂ O at 170 K is isostructural with that of [EuZnL ₃ ][ClO ₄ ][CF ₃ SO ₃ ] ₄ · 4MeCN and indicates that (i) the Fe–N bonds are in the range expected for essentially low-spin Fe ^II and (ii) [LaFeL ₃ ] ⁵⁺ adopts the regular triple-helical structure found in solution. Magnetic measurements in the solid state showed smooth spin transitions similar to those observed in solution, while photophysical studies suggested that Eu ^III →Fe ^II (low-spin) energy transfers are responsible for the complete quenching of the Eu-centred emission.