1. SSCr Meeting, Bern, 2006J-N. Chotard, Y. Filinchuk, K. Yvon La 2 MgNi 2 H 8, the first mixed polyanionic transition metal hydride J-N. Chotard, Y. Filinchuk, K. Yvon

2. Introduction Complex metal hydrides ~ 80 compounds containing ~ 30 different transition metal hydride complexes: almost all of them are mononuclear 1st polynuclear hydride complexes based on 3d-metal [Ni 2 H 7 ] 7- & [Ni 4 H 12 ] 12- SSCr Meeting, Bern, 2006J-N. Chotard, Y. Filinchuk, K. Yvon

3. Synthesis and structure determ. La 2 MgNi 2 : arc-melting La and Ni, sintering the product with Mg La 2 MgNi 2 H 8 : hydrogenation/deuteration in an autoclave: 30 bar, 100°C High-resolution synchrotron powder diffraction (SNBL at ESRF): indexing, metal structure (FOX), Rietveld High-resolution neutron powder diffraction (HRPT, SINQ, PSI): 16 deuterium atom positions (FOX), Rietveld SSCr Meeting, Bern, 2006J-N. Chotard, Y. Filinchuk, K. Yvon

4. Structure of La 2 MgNi 2 H 8 Metal atom shifts < 0.65 Å 11.84 7.82 11.96 Å 92.78 ° 7.64 3.94 Å Tetragonal Monoclinic SSCr Meeting, Bern, 2006J-N. Chotard, Y. Filinchuk, K. Yvon La 2 MgNi 2 La 2 MgNi 2 H 8

5. Two types of D-atom positions La 2 MgNi 2 La 2 MgNi 2 H 8 SSCr Meeting, Bern, 2006J-N. Chotard, Y. Filinchuk, K. Yvon

6. Bonding :18 electron rule 34 electrons 64 electrons La 3+, Mg 2+ [Ni 2 H 7 ] 7-, [Ni 4 H 12 ] 12-, H - This concept is supported by observed changes in physical properties SSCr Meeting, Bern, 2006J-N. Chotard, Y. Filinchuk, K. Yvon