Fe-Fe bonding in the rhombic Fe 4 cores of the Zintl clusters [Fe 4 E 18 ] 4- (E = Sn and Pb).

We report here the synthesis and characterization of two endohedral Zintl-ion clusters, [Fe 4 Sn 18 ] 4- and [Fe 4 Pb 18 ] 4- , which contain rhombic Fe 4 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 Fe 4 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.