Structural and energetic properties of Ir_4H_2 have been determined by applying a relativistic density functional method.As previously obtained for Ir_4H,terminal coordination of H ligands is preferred,in contrast to some other transition metals.Square-planar Ir_4 isomers with an H binding energy of up to 318 kJ mol~(-1) per atom were determined as the most stable structures of Ir_4H_2.Isomers with a tetrahedral or butterfly structure of the metal framework exhibit average H atom binding energies of up to approx 300 kJ mol~(-1).For all three types of isomers,a surprisingly large number of stable minima was identified.Unexpectedly,structural as well as energetic properties of Ir_4H_2 complexes are very similar to Ir_4H.Thus binding of an H atom to Ir_4 is only slightly affected by the presence of a second H ligand.In all cases examined,the reaction H_2 + Ir_4-> H_2Ir_4 was found to be exothermic with reaction energies of up to 170 kJ mol~(-1).
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