The Biicosahedral Complex Anions [M(B11H11)2]3- (M = Cu, Ag, Au): Synthesis and Unexpected Low-Temperature Phase Transition of [Ag(η5-B11H11)2]3- to [Ag(η2-B11H11)2]3.

We disclose the synthesis and properties of [M(B11H11)2]3- (M = Cu, Ag, Au) with the first gold-based anion [Au(B11H11)2]3-. The nido-[B11H11]4- ligand stabilizes complexes with copper, silver, and gold in the highest known oxidation state +V. The relative stability of [M+V(nido-B11X11)2]3-, [M+III(nido-B11X11)(closo-B11X11)]3-, and [M+I(closo-B11X11)2]3- isomers (X = H, F) was calculated at the DFT level. Unlike for [Cu(B11H11)2]3- and [Au(B11H11)2]3-, in case of [Ag(B11H11)2]3-, we discovered the equilibrium between the kinetically stable [Ag+V(nido-B11H11)2]3- (η5) and kinetically labile [Ag+I(closo-B11H11)2]3- (η2) 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 η5 to η2. The phase transition is reversible and upon heating to 170 K, the η2 to η5 coordination is transformed. The reaction of [M(B11H11)2]3- with HF leads to the partial substitution of hydrogen atoms by fluorine atoms to form [M(B11H11-xFx)2]3- (M = Cu, Au; x ≈ 4). We synthesized 18 compounds containing [M(B11H11)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 K3[Cu(B11H11)2]·5H2O.