Snap-shots of cluster growth: structure and properties of a Zintl ion with an Fe3 core, [Fe3Sn18]4

The endohedral Zintl-ion cluster [Fe3Sn18]4− contains a linear Fe3 core with short Fe–Fe bond lengths of 2.4300(9) Å. The ground state is a septet, with significant σ and π contributions to the Fe–Fe bonds. The Sn18 cage is made up of two partially fused Sn9 fragments, and is structurally intermediate between [Ni2CdSn18]6−, where the fragments are clearly separated and [Pd2Sn18]4−, where they are completely fused. It therefore represents an intermediate stage in cluster growth. Analysis of the electronic structure suggests that the presence of the linear Fe–Fe–Fe unit is an important factor in directing reactions towards fusion of the two Sn9 units rather than the alternative of oligomerization via exo bond formation.