Despite many exploratory studies over the past several decades, the presently known transition metals that form homoleptic transition-metal hydride complexes are limited to the Groups 7-12. Here we present evidence for the formation of Mg3CrH8, containing the first Group 6 hydride complex [CrH7]5-. Our theoretical calculations reveal that pentagonal-bipyramidal H coordination allows the formation of σ-bonds between H and Cr. The results are strongly supported by neutron diffraction and IR spectroscopic measurements. Given that the Group 3-5 elements favor ionic/metallic bonding with H, along with the current results, the true boundary for the formation of homoleptic transition-metal hydride complexes should be between Group 5 and 6. As the H coordination number generally tends to increase with decreasing atomic number of transition metals, the revised boundary suggests high potential for further discovery of hydrogen-rich materials that are of both technological and fundamental interest. [CrH7]5-: The true boundary for the formation of homoleptic transition-metal hydride complexes was demonstrated through the synthesis of Mg3CrH8 containing [CrH7]5-. Given a general trend of increasing H coordination number with decreasing atomic number of transition metals, these findings will pave the way for further discovery of hydrogen-rich materials that are of both technological and fundamental interest.
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