Abstract
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.