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Die ontdekking van die vier diatomiese digoudverbindings, Au₂, Au₂⁺, Au₂²⁺ en Au₂^–, het ’n belangrike nuwe navorsingsgebied in gasfasestudies van swaarder oorgangselemente ingelui. Hoogtepunte uit die daaropvolgende ondersoeke wat in hierdie oorsigartikel krities onder die soeklig kom, sluit in die beskrywing van die elektroniese strukture en bindingseienskappe van die betrokke spesies in die grond- sowel as opgewekte toestand, die eenduidige toekenning van sekere elektroniese oorgange gedurende foto-aktivering en verskeie sienings oor die fisiese bindingsvormingsproses vir diatomiese sisteme. Regium- en waterstofbindingsvorming word ook op ’n fundamentele vlak vergelyk. Baanbrekerstoepassings het reeds uit die gerapporteerde werk voortgevloei en sluit in aktuele, hoogs selektiewe katalitiese prosesse (o.a. CO-oksidasie en metaanaktivering) en die gepaardgaande kompleksvorming met eenvoudige monodentate toeskouer- of deelnemende ligande. Verskeie voorstelle wat belofte inhou vir verdere ondersoek in hierdie dinamiese navorsingsveld word gemaak.

Trefwoorde: metaal–metaalbindings; massaspektrometrie; ab initio-berekeninge; elektronkonfigurasies; Hund-koppelingsgevalle; spin–orbitaalkoppeling; relativistiese effekte; potensiaalkromme; valensbindingbenadering; fisiese bindingsvormingsproses; viriale teorema; energie-partisiekromme; katalise; koolmonoksiedoksidasie; metaanaktivering; watergasverskuiwingsreaksie; alkeenaktivering; regiumbinding; waterstofbinding; halogeenbinding

Abstract

Diatomic Au–Au-bonded species: multi-faceted and multifunctional

The discovery of four diatomic digold compounds, namely Au₂, Au₂⁺, Au₂²⁺ and Au₂^–, has opened a compelling frontier in gas-phase studies of third-row transition elements. This review article critically examines key findings from subsequent investigations, which encompass the elucidation of electronic structures and bonding attributes in ground and excited states for these species. Additionally, it addresses the unequivocal assignment of specific electronic transitions upon photoactivation and explores contrasting viewpoints concerning the physical processes underpinning bond formation in diatomic systems. A fundamental comparison between regium and hydrogen bonding mechanisms is presented. Notably, pioneering applications have emerged, including highly selective and relevant catalytic processes such as CO oxidation, methane activation, and dehydration, along with the formulation of appropriate catalytic cycles and complexation phenomena involving simple monodentate ligands, some of which exhibit “non-innocent” behaviour. This review also outlines several promising avenues for future research in this dynamic field.

Keywords: metal–metal bonds; mass spectrometry; ab initio calculations; electron configurations; Hund coupling cases; spin–orbit coupling; relativistic effects; potential energy curves; valence bond approach; physical chemical bond formation; virial theorem; energy-partition curves; catalysis; carbon monoxide oxidation; methane activation; water-gas shift reaction; alkene activation; regium bonding; hydrogen bonding; halogen bonding

Extended abstract

For many chemists, the simplest way to qualitatively interpret the Au–Au bond in diatomic systems is through a spin-free configurational description. Advances in computational resources have led to an increasing reliance on density functional theory (DFT), correlated quantum chemical methods, and wave function approximations for structural discussions, allowing for the consideration of intramolecular electron-electron interactions and relativistic effects, which are particularly crucial for gold atoms.

Future mass spectrometric investigations are likely to focus more on deciphering dynamic chemical interactions rather than solely identifying and characterizing compounds. Techniques such as infrared multiple-photon dissociation (IR-MPD) spectroscopy and rotational spectra analysis are expected to play a pivotal role in these endeavours.

Herein, we highlight key findings from both experimental and theoretical investigations in this domain, along with potential avenues for further exploration:

• The excitation of the lowest energy electron in neutral Au₂ results in an increase in bond length (r_e) and a decrease in the vibrational constant (ω_e). Conversely, when an electron in Au₂⁺ is selectively photo-excited, a stronger covalent bond forms, exhibiting the opposite trend.
• Despite achieving acceptable accuracy in calculating ground state binding energies and spectroscopic constants for selected digold species (Au₂, Au₂⁺, Au₂²⁺, and Au₂^–), unambiguously assigning their electronic transitions remains challenging. Even for the seemingly straightforward ion Au₂⁺, further high-level calculations are necessary.
• The digold molecular dication exists exclusively in an excited state as a unique, metastable diatomic excimer, [Au₂²⁺]*. Its formation does not follow the conventional ground state aurophilic interaction, and its decay mechanism remains unknown. Facile preparative methods are required.
• The hypothesis of electron loss during the photo-oxidation of Au₂^– from an excited Feshbach state (Scheme 4) requires further theoretical confirmation.
• Valence bond calculations for diatomic systems provide a fresh perspective on gold atom bonding, emphasizing one-electron orbital overlaps and a charge-shift resonance energy (CSRE) index. As a result, Au₂ may be described as quasi-CS bonded.
• A novel viewpoint on digold interactions emerges when considering regium bonding and hydrogen or halogen bonding with Au₂ in terms of matching surface electron density humps and holes (Figures 11–13). Experimental investigation of these bond types in digold species remains pending, and a theoretical comparison of the relative contributions of charge transfer and relativistic effects would be enlightening.
• Diverse and sometimes conflicting views on the physical process of chemical bond formation between isolated atoms exist in the literature. Key points of contention include the utility of energy decomposition schemes, the decisive role of energy type (kinetic or potential), and whether energy changes are predominantly localized around or between two participating nuclei (Scheme 3). Resolving these issues represents a significant challenge for theoreticians. Such investigations have not yet been undertaken for heavier metallic elements (including gold).
• Au₂^– and Au₂⁺ catalyse various gas-phase processes, including CO oxidation, the water-gas shift reaction (WGSR), and methane dehydration. Proposed catalytic cycles (Schemes 4–6) rely on both experimental results and theoretical calculations. Structure verification of metastable intermediates in the CO oxidation (Scheme 4) and experimental identification of intermediates in the WGSR (Scheme 5) are among the outstanding tasks. The energy source responsible for reported methane activation and consecutive endothermic dehydration to form ethene, needs to be identified.
• While 1:2 complexation of monodentate ligands with naked, active dimetal ions is known, further exploration of complex ligands with diverse properties and bonding modes is encouraged.

In conclusion, the study of dinuclear gold species presented in this review provides valuable insights that may be applied to interpret chemical changes induced by gold clusters and finely divided gold particles in the condensed phase, representing a broader challenge in the field.

• Hierdie artikel se fokusprent is geskep deur Peter Olexa en is verkry op Unsplash.

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