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

Abstract

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.