The Biicosahedral Complex Anions [M(B ₁₁ H ₁₁ ) ₂ ] ³⁻ (M = Cu, Ag, Au): Synthesis and Unexpected Low‐Temperature Phase Transition of [Ag(η ⁵ ‐B ₁₁ H ₁₁ ) ₂ ] ³⁻ to [Ag(η ² ‐B ₁₁ H ₁₁ ) ₂ ] ³⁻

We disclose the synthesis and properties of [M(B 11 H 11 ) 2 ] 3− (M = Cu, Ag, Au) with the first gold‐based anion [Au(B 11 H 11 ) 2 ] 3− . The nido ‐[B 11 H 11 ] 4− ligand stabilizes complexes with copper, silver, and gold in the highest known oxidation state +V. The relative stability of [M +V ( nido ‐B 11 X 11 ) 2 ] 3− , [M +III ( nido ‐B 11 X 11 )( closo ‐B 11 X 11 )] 3− , and [M +I ( closo ‐B 11 X 11 ) 2 ] 3− isomers (X = H, F) was calculated at the DFT level. Unlike for [Cu(B 11 H 11 ) 2 ] 3− and [Au(B 11 H 11 ) 2 ] 3− , in case of [Ag(B 11 H 11 ) 2 ] 3− , we discovered the equilibrium between the kinetically stable [Ag +V ( nido ‐B 11 H 11 ) 2 ] 3− (η 5 ) and kinetically labile [Ag +I ( closo ‐B 11 H 11 ) 2 ] 3− (η 2 ) by X‐ray diffraction. The isomerization of the Ag complex shows signs of Ag(V)–Ag(I) redox processes. When a crystal is cooled, a phase transition occurs at 110–130 K in which the coordination of the ligands on the silver changes from η 5 to η 2 . The phase transition is reversible and upon heating to 170 K, the η 2 to η 5 coordination is transformed. The reaction of [M(B 11 H 11 ) 2 ] 3− with HF leads to the partial substitution of hydrogen atoms by fluorine atoms to form [M(B 11 H 11−x F x ) 2 ] 3− (M = Cu, Au; x ≈ 4). We synthesized 18 compounds containing [M(B 11 H 11 ) 2 ] 3− (M = Cu, Ag, Au) and their derivatives, followed by structural determination (X‐ray crystallography). XANES data confirm the high oxidation state of Cu in K 3 [Cu(B 11 H 11 ) 2 ]·5H 2 O.