Bimetallic Borohydrides in the System <i>M</i>(BH₄)₂–KBH₄ (<i>M</i> = Mg, Mn): On the Structural Diversity

Four novel bimetallic borohydrides have been discovered, K₂<i>M</i>(BH₄)₄ (<i>M</i> = Mg or Mn), K₃Mg­(BH₄)₅, and KMn­(BH₄)₃, and are carefully investigated structurally as well as regarding their decomposition reaction mechanism by means of in situ synchrotron radiation powder X-ray diffraction (SR-PXD), vibrational spectroscopies (Raman and IR), thermal analysis (TGA and DTA), and ab initio density functional theory (DFT) calculations. Mechano-chemical synthesis (ball-milling) using the reactants KBH₄, α-Mg­(BH₄)₂, and α-Mn­(BH₄)₂ ensures chlorine-free reaction products. A detailed structural analysis reveals significant similarities as well as surprising differences among the two isomorphs K₂<i>M</i>(BH₄)₄, most importantly concerning the extent to which the complex anion [<i>M</i>(BH₄)₄]^2– is isolated in the structure. Anisotropic thermal expansion and an increase in symmetry at high temperatures in K₃Mg­(BH₄)₅ is ascribed to the motion of BH₄ groups inducing hydrogen repulsive effects, and the dynamics of K₃Mg­(BH₄)₅ are investigated. Decomposition in the manganese system proceeds via the formation of KMn­(BH₄)₃, the first perovkite type borohydride reported to date