MYb(BH4)4 (M = K, Na) from laboratory X-ray powder data.

Two new borohydrides, potassium ytterbium tetraborohydride, KYb(BH4)4, and sodium ytterbium tetraborohydride, NaYb(BH4)4, have been synthesized via mechanochemical reactions in the solid state. The two compounds are isostructural and both crystallize in the Cmcm space group in the structure reported previously for NaSc(BH4)4 and KY(BH4)4. This crystal structure is composed of isolated homoleptic [Yb(BH4)4](-) anions surrounded by M(+) cations (M = Na, K). The packing of the M(+) cations and [Yb(BH4)4](-) anions is a distorted variant of the hexagonal NiAs structure type, with M(+) forming distorted trigonal prisms, i.e. M6. Each second prism surrounds a [Yb(BH4)4](-) anion, while the [Yb(BH4)4](-) anions are arranged into deformed octahedra around the M(+) cations.