Dimerization of a C 2 B 3 ‐ closo ‐Carbaborane(5) to the C 4 B 6 ‐Adamantane and its 2z/3z Valence Isomerization to the C 4 B 6 ‐ nido ‐Carbaborane(10)

The competition between classical and nonclassical structures of compounds of the type C 4 B n R n+4 is supported for the first time by experimental evidence. Whether the dimerization of 1 leads to the C 4 B 6 ‐adamantane derivative 2 or to the C 4 B 6 ‐ nido ‐carbaborane(10) 3 is determined by the substituents on the C atoms