Vibrational spectroscopy of tetrahedral ternary metal hydrides: Mg2NiH4, Rb3ZnH5 and their deuteridesElectronic supplementary information (ESI) available: Force constants for the [ZnH4]2– ion in D2d symmetry (Table S1) and the [NiH4]2– ion in C1 symmetry (Table S2). See http://www.rsc.org/suppdata/cp/b1/b109975b/