The Buildup of Eight‐Vertex Tetrametallaborane Clusters: Bisdisphenoidal versus Tetracapped Tetrahedral Structures

The experimental structures of Cp 4 M 4 B 4 H 4 (M = Co, Ni) are both bisdisphenoids. However, in the cobalt compound, the metal atoms are located at the four degree 5 vertices, whereas in the nickel compound the metal atoms are located at the four degree 4 vertices. Density functional theory has shown these structures to be the lowest‐energy structures by the substantial margins of around 23 and 15 kcal/mol for M = Co and Ni, respectively. The unknown analogous tetraferraborane Cp 4 Fe 4 B 4 H 4 was predicted to have a different type of structure, namely a tetracapped tetrahedron, with the iron atoms at the four degree 6 vertices, similarly to the experimental structure of the valence isoelectronic Cp 4 Fe 4 (μ 3 ‐CO) 4 . The experimentally unknown Cp 4 M 4 B 2 H 2 and Cp 4 M 4 systems have also been optimized. Addition of BH units to either Cp 4 M 4 B 2 H 2 or Cp 4 M 4 to give the corresponding Cp 4 M 4 B 4 H 4 derivatives has been predicted to be extremely exothermic.