Transition metal–hydrogen complexes in the Mg2NiH4 and Mg2FeH6 crystals described by quantum chemical calculations

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Abstract
High accuracy quantum chemical calculations (CASSCF and contracted CI) have been performed on the transition metal hydrogen complexes in the Mg2FeH6 and Mg2NiH4 crystals. The effects from the surrounding crystal are simulated by a Madelung field. If the conventional charge of +2 is assumed for the magnesium ions the transition metal hydrogen complexes receive a charge of −4. Good agreement with experiment is then obtained for the bond distance and totally symmetric vibrational frequency for the iron complex. The calculated Fe–H distance in the octahedral FeH4−6 complex is 1.56 Å, which is identical to the value obtained from neutron diffraction data on the Mg2FeH6 crystal. For the nickel compound also the unconventional case of Mg+ has to be considered, yielding a 2− transition metal hydrogen complex. The structure for the NiH4 complex is not as well determined as the iron complex. Experimental data has been interpreted in terms of a planar structure but a tetrahedral structure is not ruled out. Our calculations show that the conformation of the complex depends on the charge it is assumed to have. A 2− complex leads to a square planar structure and a 4− complex leads to a tetrahedral structure.