A new potassium scandium borohydride, KSc(BH₄)₄, is presented and characterized by a combination of in situ synchrotron radiation powder X-ray diffraction, thermal analysis, and vibrational and NMR spectroscopy. The title compound, KSc(BH₄)₄, forms at ambient conditions in ball milled mixtures of potassium borohydride and ScCl₃ together with a new ternary chloride K₃ScCl₆, which is also structurally characterized. This indicates that the formation of KSc(BH₄)₄ differs from a simple metathesis reaction, and the highest scandium borohydride yield (~31 mol %) can be obtained with a reactant ratio KBH₄:ScCl₃ of 2:1. KSc(BH₄)₄ crystallizes in the orthorhombic crystal system, a = 11.856(5), b = 7.800(3), c = 10.126(6) Ã…, V = 936.4(8) Ã…³ at RT, with the space group symmetry Pnma. KSc(BH₄)₄ has a BaSO₄ type structure where the BH₄ tetrahedra take the oxygen positions. Regarding the packing of cations, K⁺, and complex anions, [Sc(BH₄)₄]^−, the structure of KSc(BH₄)₄ can be seen as a distorted variant of orthorhombic neptunium, Np, metal. Thermal expansion of KSc(BH₄)₄ in the temperature range RT to 405 K is anisotropic, and the lattice parameter b shows strong nonlinearity upon approaching the melting temperature. The vibrational and NMR spectra are consistent with the structural model, and previous investigations of the related compounds ASc(BH₄)₄ with A = Li, Na. KSc(BH₄)₄ is stable from RT up to ~405 K, where the compound melts and then releases hydrogen in two rapid steps approximately at 460−500 K and 510−590 K. The hydrogen release involves the formation of KBH₄, which reacts with K₃ScCl₆ and forms a solid solution, K(BH₄)_1−xCl_x. The ternary potassium scandium chloride K₃ScCl₆ observed in all samples has a monoclinic structure at room temperature, P2₁/a, a = 12.729(3), b = 7.367(2), c = 12.825(3) Ã…, β = 109.22(2)°, V = 1135.6(4) Ã…³, which is isostructural to K₃MoCl₆. The monoclinic polymorph transforms to cubic at 635 K, a = 10.694 Ã… (based on diffraction data measured at 769 K), which is isostructural to the high temperature phase of K₃YCl₆.

A new alkaline transition-metal borohydride, NaSc(BH₄)₄, is presented. The compound has been studied using a combination of in situ synchrotron radiation powder X-ray diffraction, thermal analysis, and vibrational and NMR spectroscopy. NaSc(BH₄)₄ forms at ambient conditions in ball-milled mixtures of sodium borohydride and ScCl₃. A new ternary chloride Na₃ScCl₆ (P2₁/n, a = 6.7375(3) Ã…, b = 7.1567(3) Ã…, c = 9.9316(5) Ã…, β = 90.491(3)°, V = 478.87(4) Ã…³), isostructural to Na₃TiCl₆, was identified as an additional phase in all samples. This indicates that the formation of NaSc(BH₄)₄ differs from a simple metathesis reaction, and the highest scandium borohydride yield (22 wt %) was obtained with a reactant ratio of ScCl₃/NaBH₄ of 1:2. NaSc(BH₄)₄ crystallizes in the orthorhombic crystal system with the space group symmetry Cmcm (a = 8.170(2) Ã…, b = 11.875(3) Ã…, c = 9.018(2) Ã…, V = 874.9(3) Ã…³). The structure of NaSc(BH₄)₄ consists of isolated homoleptic scandium tetraborohydride anions, [Sc(BH₄)₄]^–, located inside slightly distorted trigonal Na₆ prisms (each second prism is empty, triangular angles of 55.5 and 69.1°). The experimental results show that each Sc³⁺ is tetrahedrally surrounded by four BH₄ tetrahedra with a 12-fold coordination of H to Sc, while Na⁺ is surrounded by six BH₄ tetrahedra in a quite regular octahedral coordination with a (6 + 12)-fold coordination of H to Na. The packing of Na⁺ cations and [Sc(BH₄)₄]^– anions in NaSc(BH₄)₄ is a deformation variant of the hexagonal NiAs structure type. NaSc(BH₄)₄ is stable from RT up to ∼410 K, where the compound melts and then releases hydrogen in two rapidly occurring steps between 440 and 490 K and 495 and 540 K. Thermal expansion of NaSc(BH₄)₄ between RT and 408 K is anisotropic, and lattice parameter b shows strong anomaly close to the melting temperature.