Fe–Fe bonding in the rhombic Fe4 cores of the Zintl clusters [Fe4E18]4− (E = Sn and Pb)

Abstract

We report here the synthesis and characterization of two endohedral Zintl-ion clusters, [Fe₄Sn₁₈]⁴⁻ and [Fe₄Pb₁₈]⁴⁻, which contain rhombic Fe₄ cores. The Fe–Fe bond lengths are all below 2.5 Å, distinctly shorter than in the corresponding Cu clusters, indicating the presence of Fe–Fe bonding. Subtle differences in the structure of the Fe₄ core between the two clusters suggest that the change in tetrel element causes a change in electronic ground state, with a very short Fe–Fe bond length of 2.328 Å present across the diagonal of the rhombus in the lead case.