Snap-shots of cluster growth: structure and properties of a Zintl ion with an Fe core, [FeSn]

The endohedral Zintl-ion cluster [Fe 3 Sn 18 ] 4− contains a linear Fe 3 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 Sn 18 cage is made up of two partially fused Sn 9 fragments, and is structurally intermediate between [Ni 2 CdSn 18 ] 6− , where the fragments are clearly separated and [Pd 2 Sn 18 ] 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 Sn 9 units rather than the alternative of oligomerization via exo bond formation. The effective oxidation state of the Sn 18 unit determines the degree of fusion of the two Sn 9 polyhedra in the series [Ni 2 CdSn 18 ] 6− , [Fe 3 Sn 18 ] 4− and [Pd 2 Sn 18 ] 4− . The new [Fe 3 Sn 18 ] 4− cluster represents an intermediate phase in the fusion process.