A new sodium-yttrium borohydride-chloride, NaY(BH4)(2)Cl-2, is obtained by a combination of mechanochemical synthesis and annealing of NaBH4-YCl3 mixtures and is characterized by in-situ synchrotron radiation X-ray powder diffraction, density functional theory, thermal analysis and vibrational spectroscopy. Several simultaneous and coupled reactions occur during the synthesis, also yielding Na3YCl6 and Na(BH4)(1-x)Cl-x besides the title compound. The polymeric pseudo-orthorhombic crystal structure of NaY(BH4)(2)Cl-2 (space group P2/c) is built of edge- and corner-sharing octahedral coordination polyhedra of yttrium (4Cl + 2BH(4)) and sodium (2Cl + 4BH(4)). The structure is isomorphous to the high temperature polymorph of NaYCl4. The BH4 units in NaY(BH4)(2)Cl-2 are located only on the larger of the two independent anion sites in NaYCl4. Density functional theory optimization of the experimental structure suggests that the BH4 units act as eta(3)-ligands (face-sharing) towards yttrium and eta(1)-ligands (corner-sharing) towards sodium. Raman spectroscopy confirms this BH4 configuration. NaY(BH4)(2)Cl-2 decomposes at similar to 300 degrees C under formation of Na3YCl6, while the latter compound at higher temperatures reacts with Na(BH4)(1-x)Cl-x to form NaCl and possibly amorphous products. The reactions are associated with mass losses of 2.62 and 3.78 wt% for the NaBH4-YCl3 (3:1) and (4:1) samples, respectively. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.