Abstract
Group 11 dihydrides MH2– (M = Cu, Ag, Au, Rg) have been much less studied than the corresponding MH compounds, despite having potentially several interesting applications in chemical research. In this work, their main spectroscopic constants (bond lengths, dissociation energies, and force constants) have been evaluated by means of highly accurate relativistic four-component coupled cluster (4c-CCSD(T)) calculations in combination with large basis sets. Periodic trends have been quantitatively explained by the charge-displacement/natural orbitals for chemical valence (CD-NOCV) analysis based on the four-component relativistic Dirac–Kohn–Sham method, which allows a consistent picture of the nature of the M–H bond to be obtained on going down the periodic table in terms of Dewar–Chatt–Duncanson bonding components. A strong ligand-to-metal donation drives the M–H bond and it is responsible for the heterolytic (HM···H–) dissociation energies to increase monotonically from Cu to Rg, with RgH2– showing the strongest and most covalent M–H bond. The “V”-shaped trend observed for the bond lengths, dissociation energies, and stretching frequencies can be explained in terms of relativistic effects and, in particular, of the relativistically enhanced sd hybridization occurring at the metal, which affects the metal–ligand distances in heavy transition-metal complexes. The sd hybridization is very small for Cu and Ag, whereas it becomes increasingly important for Au and Rg, being responsible for the increasing covalent character of the bond, the sizable contraction of the Au–H and Rg–H bonds, and the observed trend. This work rationalizes the spectroscopic/bond property relationship in group 11 dihydrides within highly accurate relativistic quantum chemistry methods, paving the way for their applications in chemical bond investigations involving heavy and superheavy elements.
Highlights
Silver, and gold hydrides (MH compounds, otherwise referred to as “coinage metal’ hydrides” and hereafter reported as “monohydrides”) have been known for more than a century (the first example of polymeric Cu(I) hydride is from the 1840s),[1] and nowadays, there is plenty of literature concerning their characterization.[2−9] The great interest for these compounds is motivated by the range of applications they have in chemical research, such as in homogeneous catalysis[10] or in the field of renewable energies, where they could be used for hydrogen storage.[11−14] In addition, the study of the M−H bond in these species is highly appealing for the exploration of relativistic effects in chemistry
The first part of this work is devoted to the evaluation of the spectroscopic constants of the MH2− (M = Cu, Ag, Au, Rg) complexes via highly accurate four-component relativistic approaches
We already pointed out that an accurate inclusion of both relativistic effects and electron correlation affects bond lengths in MH compounds,[25] and it is reasonable to expect the same impact for MH2− complexes
Summary
Silver, and gold hydrides (MH compounds, otherwise referred to as “coinage metal’ hydrides” and hereafter reported as “monohydrides”) have been known for more than a century (the first example of polymeric Cu(I) hydride is from the 1840s),[1] and nowadays, there is plenty of literature concerning their characterization.[2−9] The great interest for these compounds is motivated by the range of applications they have in chemical research, such as in homogeneous catalysis[10] or in the field of renewable energies, where they could be used for hydrogen storage.[11−14] In addition, the study of the M−H bond in these species is highly appealing for the exploration of relativistic effects in chemistry. The relevance of using these species as probes for relativistic effects grew even more after 1994, when the heavier homologue of gold, roentgenium (Rg), was artificially synthesized.[21] As roentgenium is a superheavy element, relativistic effects (scalar and spin−orbit coupling) are expected to have a huge impact on its chemical behavior, even to a greater extent with respect to gold. The only viable way to explore the chemical behavior of this superheavy element is through theoretical calculations, which must include the relativistic effects (and electron correlation) at the highest accuracy.
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