The Elusive [B(ReO 4 ) 4 ] − Anion in the Non‐Centrosymmetric Crystal Structure of K[B(ReO 4 ) 4 ]

The reaction of KReO 4 with B 2 O 3 in molten Re 2 O 7 results in the formation of a colorless crystalline powder of K[B(ReO 4 ) 4 ]. The single‐crystal structure determination reveals a non‐centrosymmetric structure (space group I ), containing the hitherto unknown [B(ReO 4 ) 4 ] − anion. This anion presents the first example of a monomeric anion with boron as a central atom coordinated by four perrhenate anions forming B─O─Re bonds. The compound has been further characterized by Raman spectroscopy, with additionally DFT calculations to assign the vibration modes. Moreover, the absence of inversion symmetry has been proven by second harmonic generation (SHG) measurements.