B4 Tetrahedra for Aluminum Atoms—A Surprising Substitution in τ-Borides Ni20Al3B6 and Ni20AlB14

Stabilized by the tailor-made coordination in the crystal , B₄ tetrahedra were characterized for the first time in a solid-state structure (shown on the right). Some of the Al atoms are replaced by B₄ tetrahedra in the cubic τ-boride Ni₂₀AlB₁₄. The B–B distances of 1.681(15) Å are similar to those in the molecular structures of B₄Cl₄ and B₄(tBu)₄, which contain tetrahedral B₄ units.