Snap-shots of cluster growth: structure and properties of a Zintl ion with an Fe 3 core, [Fe 3 Sn 18 ] 4 .

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.