Two novel bimetallic borohydrides, Rb[Y(BH₄)₄] and Cs[Y(BH₄)₄], have been synthesised via mechanochemical reactions between MBH₄, M = Rb or Cs and Y(BH₄)₃. Both are ionic compounds comprising complex anions [Y(BH₄)₄]⁻ and alkali metal cations, and they adopt crystal structures which were not observed previously for quasi-ternary borohydrides. LiCl, a by-product from the synthesis of Y(BH₄)₃, is not an unreactive dead-weight but rather it promotes formation of trimetallic borohydride-chlorides, M₂Li[Y(BH₄)_6−xCl_x], which crystallise in the elpasolite-type structures. Formation of Cs₂Li[Y(BH₄)_6−xCl_x] takes place during the mechanochemical synthesis of Cs[Y(BH₄)₄] at room temperature, while that of Rb₂Li[Y(BH₄)_6−xCl_x] only after heating of the sample above the melting point. The M[Y(BH₄)₄]/M₂Li[Y(BH₄)_6−xCl_x] mixtures slowly release H₂ when melted (T_m = 170 °C for M = Rb, 210 °C for M = Cs) but the main stage of thermal decomposition occurs at a T_dec of ∼270 °C, similarly to what is observed for K[Y(BH₄)₄] and NaBH₄/Y(BH₄)₃ composites. We also demonstrate how the T_dec of M[Y(BH₄)₄] salts may be tuned by choosing the MBH₄ precursor of the appropriate Lewis basicity.