[Co₂ @Ge₁₆ ]^4- : Localized versus Delocalized Bonding in Two Isomeric Intermetalloid Clusters

We report the successful isolation and structural elucidation of two bimetallic doped [Co₂ @Ge₁₆ ]^4- clusters (α and β form), which were synthesized through the reaction of [{(ArN)₂ CtBu}Co(η⁶ -toluene)] (Ar=2,6-diisopropylphenyl) and K₄ Ge₉ in ethylenediamine (en) solution and co-crystallized together in [K(2,2,2-crypt)]₄ [Co₂ @Ge₁₆ ]⋅en. The α-[Co₂ @Ge₁₆ ]^4- isomer prefers a distinct D_2h 3-connected architecture, whereas the deltahedral isomeric β-[Co₂ @Ge₁₆ ]^4- isomer adopts a quasi-C_2h geometry and can be seen as coupling of two distorted arachno-[Co@Ge₁₀ ] units. Chemical bonding analyses indicate that the skeleton of the α isomer is mainly composed of localized bonds, whereas only multicenter bonding interactions govern the geometry of the β isomer, which was further found to exhibit a fluxional behavior. The coexistence of both isomers within one unit cell links the 3-connected clusters with their deltahedral congeners, thus highlighting the structural and electronic flexibility of such discreet cluster systems.