Signature of Au as a Halogen

Abstract

Gold, although chemically inert in its bulk state, is reactive at the nanoscale and, in small clusters, even behaves like a hydrogen atom. Using a photoelectron spectroscopy experiment and first-principles theory, we show that Au also behaves like a halogen in small clusters. This is evident not only in strong resemblance between the photoelectron spectra of Au 2 F − and AuF 2 − but also in Au exhibiting one of the signature properties of halogens, its ability to form superhalogens with electron affinities higher than that of any halogen atom. For example, the electron affinity (EA) of Au 2 F − is 4.17 eV, while AuF 2 − , a known superhalogen, has an EA of 4.47 eV. Of particular interest is Au 2 F 2 , which, in spite of being a closed-shell system, is a pseudohalogen with an EA of 3.3 ± 0.1 eV. Here, one of the Au atoms behaves like a halogen, making Au 2 F 2 mimic the property of AuF 3 .G old, one of the most precious metals in the periodic table, has many faces. This is due to the relativistic effect, which stabilizes its 6s orbital while destabilizing its 5d orbitals. Chemically a transition metal belonging to the group 11 elements, Au, unlike its congeners Cu and Ag, is unreactive under ambient conditions and is considered to be a noble metal. However, gold is reactive at the nanometer length scale 1 and serves as an excellent catalyst. Analogous to transition metals, Au also exhibits multiple valences, with oxidation states ranging from −1 to +6, 2−5 oxidation states of +1 and +3 being the most prevalent. However, unlike transition metals, Au is not magnetic. Like hydrogen, Au has one electron in its outermost s shell. The fact that Au and H behave alike in small clusters was first suggested by Buckart et al., 6 who observed nearly identical features in the photoelectron spectra (PES) of Au n − and Au n−1 H − (n > 2). A similar conclusion was later reached by Wang and co-workers 7−9 when they examined the PES of binary silicon−gold clusters SiAu n (n = 2−4) and found these to be analogous to those of SiH n clusters. The gold− hydrogen analogy was also suggested by Ghanty, 10 who found similarities between new rare gas auride species and the corresponding hydrides. Later, the author 11 showed that the correspondence of the electronic structure, bonding, and stability of a new class of compounds AuBX (X = F, Cl, or Br) to those of HBX radicals is one to one.Note that Au and halogen atoms also have something in common; addition of an extra electron completes the 6s shell of the Au atom as it does the outer p shell of the halogen atoms. Indeed, with the exception of halogen atoms, the Au atom has the highest electron affinity (EA), namely, 2.31 eV, of any element in the periodic table. Au forms a salt with the Cs metal, with Cs carrying a positive charge and Au, like halogens,

Au as a Surrogate for F: The Case of UAu6 vs UF6

“…Here, we adopt an alternative approach to better understand UF 6 and UF 6 – by using Au as a surrogate for F. Note that the EA of the Au atom (2.32 eV) is the highest among all metals in the periodic table; it is also one of the few metals that forms ionic bonds with metal atoms, such as Cs and Th. − That Au may mimic F in UF 6 is consistent with Hoffmann’s isolobal principle, which suggests that species with similar symmetry and frontier orbital energies have similar bonding and reactivity . It was previously predicted by theory that Au could behave like a halogen when interacting with U in UAu 4 . , Recent joint experimental–theory studies on various thorium–gold anions, as well as the Au 2 F molecule, have shown that indeed Au can behave very similarly to F. , In a computational pilot study of the UAuF trimer, we also found that Au mimics the chemistry of F (Section S2, and Figure S2 of the Supporting Information, SI).…”

“…7 It was previously predicted by theory that Au could behave like a halogen when interacting with U in UAu 4 . 8,9 Recent joint experimental−theory studies on various thorium−gold anions, as well as the Au 2 F molecule, have shown that indeed Au can behave very similarly to F. 10,11 In a computational pilot study of the UAuF trimer, we also found that Au mimics the chemistry of F (Section S2, and Figure S2 of the Supporting Information, SI).…”

Unusual Spin States in Noble Gas Inserted Noble Metal Halocarbenes, FNgCM (Ng = Kr, Xe, Rn; M = Cu, Ag, Au)

“…Moreover, the experimentally determined FKrCF and FXeCF molecules and the theoretically predicted FNgCX (Ng = Kr and Xe, X = Cl, Br, and I) molecules are also found to be stable in the singlet state owing to the strong nucleophilic nature of X atoms. Therefore, a one-to-one correspondence is obtained between Au and X atoms while comparing the relative stabilities of the singlet and triplet FNgCX and FNgCM molecules, supporting the “gold-halogen” analogy. − Accordingly, from now onward, we will discuss all the results for 3 FNgCCu, 3 FNgCAg, and 1 FNgCAu molecules, since they are the most stable state as per the results obtained by employing the above-mentioned high level computational calculations.…”

“…Hence, the behavior of the Au atom in FNgCAu coincides well with the behavior of a halogen atom (X) in FNgCX molecules and the calculated results for FNgCX (X = Br and I) and FNgCAu molecular systems are very close to each other. The presence of strong relativistic effects in the Au atom as compared to the H, Cu, and Ag atoms drive the Au atom to behave as a better electron donor, thereby stabilizing the singlet carbene and also emphasizing on the “gold-halogen” analogy. − The absence of such a behavior in hydrogen and other noble metal congeners mark a higher stability of the triplet FNgCM (M = H, Cu, and Ag) molecules.…”

“…12 However, only very recently, direct experimental evidences are provided on the "gold-halogen" analogy and reported in leading journals. 13,14 Excluding halogen atoms, Au atom has the highest electron affinity (EA) value (2.308 eV) in the periodic table. Most importantly, nearly half of the EA value of Au is attributed to the relativistic effect, since the nonrelativistic EA is close to the EA of the Ag atom (1.304 eV).…”