Structure and bonding in a bimetallic endohedral cage, [Co2@Ge16]z-

We report here mass spectrometric evidence for the new bimetallic cluster, [Co-2@Ge-16](z) which is observed as a mono-anion in the mass spectrum but as a tetra-anion in the solid state. Density functional theory suggests that at both oxidation levels it adopts a D-2h-symmetric structure based on two pentagonal bipyramids. The 3-connected vertices of the cage are characteristic of an electron-precise (5n = 80) count, but assigning 80 of the 86 valence electrons to the cage would imply an unrealistic degree of charge separation. We resolve this dilemma by showing that many of the electron pairs fulfill a dual role, contributing to the stable 80-electron count at the cage and to the 18-electron count at each metal. Thus it is not possible in this case to partition the charge into mutually exclusive subsets on the metal and on the cluster. (C) 2015 Elsevier B.V. All rights reserved.