Degrees of Distortion: Synthesis, Structure and Bonding in Approximately Icosahedral [Ru@Sn12 ] 4-

We report here the synthesis and structural characterisation of the [Ru@Sn12]4- anion, a new member of the icosahedral M@E12 family of endohedral Zintl clusters. The cluster is approximately icosahedral, but a signicant variance in the Ru-Sn and Sn-Sn bond lengths is a result of back-bonding from the Ru center to the Sn12 cage. We use density functional theory to place the structural chemistry of [Ru@Sn12]4- into the wider context of the M@E 12 family.