Is It Possible To Prepare and Stabilize Triple-Bonded Thallium≡Antimony Molecules Using Substituents?

The M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, and B3LYP/LANL2DZ+dp levels of theory were used to investigate the effect of substituents on the stability of the triple-bonded RTl≡SbR molecule. For comparison, small groups (F, OH, H, CH3, and SiH3) and sterically bulky substituents, (Ar* (=C6H3-2,6-(C6H2-2,4,6-i-Pr3)2), Tbt (=C6H2-2,4,6-{CH(SiMe3)2}3), SiiPrDis2, and SiMe(SitBu3)2), were chosen for the present study. The density functional theory results indicate that the triple-bonded RTl≡SbR compounds with small ligands are transient intermediates, so their experimental detections should be extremely difficult. Nevertheless, the theoretical observations demonstrate that only the bulkier ligands can effectively stabilize the central Tl≡Sb triple bond. In addition, the valence-electron bonding model reveals that the bonding characters of the triple-bonded RTl≡SbR species possessing sterically bulky groups can be represented as RTl ← SbR. Nevertheless, on the basis of the natural resonance theory, the natural bond orbital, and the charge decomposition analysis, the theoretical observations suggest that the Tl≡Sb triple bond in the acetylene analogues, RTl≡SbR, should be very weak.

Introduction

Due to their usage in studying fundamental chemistry problems as well as potential applications, the synthesis and isolation of heavy homonuclear alkyne analogues, RE14≡E14R (E14 = group 14 elements), have been attracting significant interest for at least 30 years.[1−28] Recently, the interests in synthesis have turned into preparing heteronuclear ethyne analogues, RC≡E14R, in which one of the carbon atoms in acetylene is substituted by one of the heavy group 14 elements (Si, Ge, Sn, and Pb).[29−36] From the valence-electron viewpoint, the heavier acetylene analogues, RE13≡E15R (E13 = B, Al, Ga, In, and Tl and E15 = N, P, As, Sb, and Bi) is isoelectronic to HC≡CH. Unfortunately, to date, the study of such acetylene analogues with groups 13 and 15 elements is still lacking from both experimental and theoretical viewpoints.[37−41]

In recent years, it has been reported that optoelectronic devices based on thallium–E15 have advantages over E12–E16 (E12 = group 12 elements and E16 = group 16 elements) semiconductor devices because it has been discovered that most Tl–E15 compounds have a very narrow band gap.[42−51] In fact, many ternary molecules possessing heteroatomic anions from E1–Tl–E15 (E1 = group 1 elements) main group species have been prepared and isolated.[52−63] Of which, for instance, a series of novel metallic compounds with the E1–Tl–Sb form have been discovered and structurally characterized.[64−68] Nevertheless, to the best of our knowledge, until now no molecules bearing the Tl≡Sb triple bond, which is isoelectronic to acetylene, have been examined either experimentally or theoretically.

It is the possibility of forming the triple-bonded RTl≡SbR molecules with the kinetic stability from the substituent viewpoints that has aroused our interest for the present study. For comparison, two kinds of ligands have been chosen in the present study. The small groups (such as F, OH, H, CH3, and SiH3) have been chosen in this work. On the other hand, the sterically bulky substituents[69−71] (such as Ar* = C6H3-2,6-(C6H2-2,4,6-i-Pr3)2), Tbt = C6H2-2,4,6-{CH(SiMe3)2}3), SiiPrDis2, and SiMe(SitBu3)2), which are given in Scheme , have been used in this study. Three kinds of density functional theory (DFT) have been applied in this study. They are M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, and B3LYP/LANL2DZ+dp methods (Supporting Information). The theoretical results will convince experimental chemists that a molecule that contains a Tl≡Sb triple bond is not an unrealistic proposition, and it is expected that successful schemes for the synthesis and isolation of these triple-bonded species will be devised soon.

Scheme 1

Results and Discussion

Small Ligands on Substituted RTl≡SbR

In this section, we first use the intramolecular 1,2-shift reaction to explore the effects of small ligands (R = CH3, H, F, OH, and CH3) on the relative stability of the RTlSbR species including the triple-bonded and double-bonded isomers. That is, RTl≡SbR → TS1 → R2Tl=Sb: and RTl≡SbR → TS2 →: Tl=SbR2. The DFT results for three methods mentioned earlier are collected in Figure .

Figure 1

M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, and B3LYP/LANL2DZ+dp surfaces for RTl≡SbR (R = F, OH, H, CH3, and SiH3). These Gibbs free energies are in kcal/mol. For details see the text.

As seen in Figure , all the theoretical results coming from the three DFT investigations reveal that the triple-bonded RTl≡SbR compounds should be unstable on the energy surfaces of the 1,2-shifted migration reactions, regardless of the electronegative or electropositive substituents. In other words, once the triple-bonded RTl≡SbR compound possessing small substituents (R) is formed, it can readily undergo the 1,2-migration to yield other kind of double-bonded isomer.

Nevertheless, referring to Figure , one may find that some of the barriers for the 1,2-shifted migration reactions are fairly large, in excess of 20 kcal/mol, in both directions for substituents such as H, Me, and SiH3. In spite of the fact that we did not consider the solvent effect for these species in this work, the present computational results imply that the triple-bonded RTl≡SbR molecules bearing the above small ligands might exist in the gas phase. However, this prediction needs to be greatly verified by the more sophisticatedly theoretical methods. Unfortunately, this is beyond the scope of the present study. Accordingly, the above theoretical analyses demonstrate that the triple-bonded RTl≡SbR compounds bearing the small ligands should not be detected experimentally as well as isolated in the laboratory, even in a low-temperature inert matrix. This theoretical conclusion is the same as in the previous cases found for RB≡SbR,[72] RAl≡SbR,[73] RGa≡SbR,[74] and RIn≡SbR[75] systems bearing small groups (R). In consequence, all of these theoretical observations strongly suggest that no matter what kind of electronegativity for the small ligands (R) are attached, the triple-bonded RE13≡SbR molecules cannot exist in the normal conditions. Therefore, we turn our attention on the effect of bulky ligands on the triple-bonded thallium≡antimony compounds in the next section.

Large Ligands on Substituted R′Tl≡SbR′

In this section, we use the bulkier substituents to see whether they can be kinetically stabilized as a monomer. Like the cases of small groups on substituted RTl≡SbR, we use four bulky ligands (R′= Ar*, Tbt, SiiPrDis2, and SiMe(SitBu3)2; Scheme )[69−71] to explore the relative energies between the triple-bonded R′Tl≡SbR′ species and its corresponding double-bonded isomers. Namely, R′Tl≡SbR′ → R2′Tl=Sb (ΔH1) and R′Tl≡SbR′ → Tl=SbR2′ (ΔH2), as shown in Scheme . On the dispersion-corrected M06-2X/Def2-TZVP level of theory,[76] the theoretical results are shown in Table . It is apparent to see that both reaction enthalpies (ΔH1 and ΔH2) are predicted to be quite high (>82 kcal/mol). The theoretical evidence, therefore, suggests the Tl≡Sb triple bond can be greatly stabilized when it is attached by two bulkier ligands.

Scheme 2

Table 1

Using Bulky Ligands and the Dispersion-Corrected M06-2X/Def2-TZVP Level of Theorya

R′	SiMe (SitBu3)2	SiiPrDis2	Tbt	Ar*
TlαSb (Å)	2.764	2.779	2.809	2.822
∠R′–Tl–Sb (deg)	161.5	162.6	168.8	167.2
∠Tl–Sb–R′ (deg)	107.1	118.1	110.8	108.1
∠R′–Tl–Sb–R′ (deg)	174.1	175.9	178.9	172.5
QTl′	0.7997	0.7336	0.9249	0.9821
QSb′	–0.4509	–0.5984	–0.0599	–0.0355
ΔETl′ for Tl–R′ (kcal/mol)b	33.92	35.52	37.27	38.24
ΔESb′ for Sb–R′ (kcal/mol)c	–16.91	–15.88	–21.31	–16.70
HOMO–LUMO (kcal/mol)	49.41	45.68	46.24	67.01
BE (kcal/mol)d	69.21	79.61	83.38	67.54
ΔH1 (kcal/mol)e	95.57	96.86	90.78	94.70
ΔH2 (kcal/mol)e	82.01	86.55	91.19	90.45
WBIf	2.147	2.214	2.017	2.038

The bond lengths (Å), bond angels (deg), natural charge densities (QTl′ and QSb′), singlet–triplet energy splitting (ΔETl′ and ΔESb′), binding energies (BEs), the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) energy gaps (kcal/mol), the Wiberg bond index (WBI), and some reaction enthalpies (kcal/mol) for R′Tl≡SbR′.

ΔETl′ (kcal/mol) = E (triplet state for R′–Tl) – E (singlet state for R′–Tl).

ΔESb′ (kcal/mol) = E (triplet state for R′–Sb) – E (singlet state for R′–Sb).

BE (kcal/mol) = E (triplet state for R′–Tl) + E (triplet state for R′–Sb) – E (singlet for R′Tl≡SbR′).

See Scheme .

The WBI for the Tl≡Sb bond: see refs (77, 78).

As seen in Table , the Tl≡Sb triple bond length is predicted to be 2.764–2.822 Å based on the present theoretical calculations. In fact, these computed Tl≡Sb triple-bond distances are quite similar to the estimated value (277 pm) based on the specific triple-bond covalent radii reported by Pyykkö and co-workers.[79,80]

Moreover, Table indicates that the ∠R′–Tl–Sb and ∠Tl–Sb–R′ bond angles are about 160 and 110°, respectively. These computational values are different from those of the ∠R–Tl–Sb and ∠Tl–Sb–R bond angles for the RTl≡SbR molecules bearing small substituents (Table S1). The reason for this is still uncertain. Presumably, it could be due to sterically congested systems.

From Table , the DFT calculations foresee that the R′–Tl unit has the singlet ground state, whereas the R′–Sb moiety has the triplet ground state. In particular, the DFT results demonstrate the ΔETl′ of the R′–Tl component is evaluated to be larger than 34 kcal/mol, whereas the modulus of ΔESb′ for the R′–Sb fragment is estimated to be larger than 16 kcal/mol. These theoretical results suggest the bonding characters of the triple-bonded R′Tl≡SbR′ species can be diagrammatically represented in Figure [II], which somewhat differs from Figure [I].[81,82] In other words, the triplet R′–Tl unit and the triplet R′–Sb moiety can combine to produce a triple-bonded R′Tl≡SbR′ molecule at the singlet ground state, i.e., [R′–Tl]3 + [R′–Sb]3 → [R′Tl≡SbR′]1. From Figure , the bonding picture based on the valence bond theory strongly suggests model [II], , is better to interpret the nature of the Tl≡Sb triple bond in the R′Tl≡SbR′ molecule. Namely, the Tl≡Sb triple bond in the R′Tl≡SbR′ species can be considered as one σ-bond, one π-bond, and one donor–acceptor π-bond (R′Tl ← SbR′). Again, two factors can influence the bond orders of the triple bonds in the R′Tl≡SbR′ species. As seen in Figure , one is the lone pair of orbitals of both R′–Sb and R′–Tl components consist of the valence s and p orbitals. The other is the covalent radii of Tl (148 pm) and Sb (140 pm) are different,[83] which, in turn, can make small overlap between the two heteroatoms. Moreover, due to the sterically congested substituents (R′) being attached to Tl and Sb atoms, these would again make the overlap between them to be much smaller. As a result, one may easily conceive that the bond order of the Tl≡Sb triple bond in the R′Tl≡SbR′ bearing bulky groups should not be strong. Indeed, the calculated Wiberg bond index (WBI) collected in Table suggests all the bond orders of the Tl≡Sb triple bonds are predicted to be about 2.1. Fortunately, these bond orders are much larger than the WBI values given in Table S1 for the systems of small ligands on substituted RTl≡SbR.

Figure 2

Valence-bond bonding models [I] and [II] for the triple-bonded RTl≡SbR molecule.

After submitting this paper, one reviewer proposed a question why the larger substituents produce a triple bond (Table ), whereas smaller groups clearly give a double bond (Table S1). Presumably, the reason for this could be due to the nature of the two components (R–Tl and R–Sb) in RTl≡SbR possessing small substituents as well as two units (R′–Tl and R′–Sb) in R′Tl≡SbR′ featuring bulky ligands. As can be seen in Table S1, the modulus of ΔETl for R–Tl is substantially larger than that (|ΔESb|) for R–Sb. As a result, the bonding nature of the triple-bonded RTl≡SbR molecule prefers to adopt model [I], (Figure ). That is, one donor–acceptor σ-bond (RTl → SbR) and two donor–acceptor π-bonds (RTl ← SbR). However, this Tl–Sb triple bond in the RTl≡SbR species with small groups is quite weak, showing the double-bond character as represented in Table S1. On the other hand, according to the above discussion, the bonding nature of R′Tl≡SbR′ follows model [II], . Therefore, from Figure , due to the overlap populations in model [II] being larger than those in model [I], the bulky groups generate a triple bond, whereas smaller ligands yield a double bond. This anticipation has been confirmed by the present computational data given in Tables and S1, respectively.

Besides these, as already shown in Table S1, three DFT computational data show that the Tl≡Sb triple bond length (Å) in RTl≡SbR is in the region of 2.630–2.758 (M06-2X/Def2-TZVP), 2.621–2.727 (B3PW91/Def2-TZVP), and 2.622–2.757 (B3LYP/LANL2DZ+dp), whose WBIs are predicted to be 0.988–1.394, 1.080–1.466, and 1.129–1.492, respectively. It has to be emphasized here that from Table S1, their dissociation energies are predicted to be small, less than 40 kcal/mol, for a Tl≡Sb triple bond. This is very much smaller than that in acetylene. That is to say, the theoretical evidences demonstrate that the RTl≡SbR molecules with small ligands are very unstable. A similar situation can be noticed in some other metal–metal bonds such as the Ga≡Ga triple bond in question. Some interesting works concerning the Ga≡Ga triple bond can be found in refs (84−92).

The charge decomposition analysis (CDA)[93] is utilized to interpret the interactions between SiMe(SitBu3)2–Tl and SiMe(SitBu3)2–Sb units. As seen in Table , the largest contributions from a SiMe(SitBu3)2–Sb fragment to a SiMe(SitBu3)2–Tl fragment is no. 176 (HOMO) orbital, demonstrating that the former donates electrons (0.0801 e) to the latter mainly through the HOMO orbital. Also, the largest contributions from a SiMe(SitBu3)2–Tl component to a SiMe(SitBu3)2–Sb component is no. 175 (HOMO – 1) orbital, indicating that the former donates electrons (0.0986 e) to the latter principally through the HOMO – 1 orbital. After considering the electron donations from various orbital of both SiMe(SitBu3)2–Sb and SiMe(SitBu3)2–Tl, one may obtain the net amount of electron transfer, which is calculated to be negative (−0.728), suggesting that a SiMe(SitBu3)2–Sb unit donates more electrons to a SiMe(SitBu3)2–Tl unit. This result based on the CDA computations is in good agreement with model [II] (Figure ). The computational CDA results for the other three groups (R′= SiiPrDis2, Tbt, and Ar*) are collected in the Supporting Information. Again, these theoretical evidences demonstrate that the bonding character of R′Tl≡SbR′ is represented as .

Table 2

Charge Decomposition Analysis for SiMe(SitBu3)2–Tl≡Sb–SiMe(SitBu3)2 Based on the Dispersion-Corrected M06-2X/Def2-TZVP Methoda,b

	orbital	occupancy	A	B	A – B	W
	166	2.000000	0.001058	0.004709	–0.003652	–0.009191
	167	2.000000	0.000123	0.000578	–0.000455	–0.004267
	168	2.000000	0.000701	0.003343	–0.002642	–0.001491
	169	2.000000	0.001787	0.002765	–0.004552	–0.002579
	170	2.000000	0.000864	0.001693	–0.000829	–0.010902
	171	2.000000	0.001902	0.004846	–0.002944	–0.013524
	172	2.000000	0.004147	0.009106	–0.004959	–0.008967
	173	2.000000	0.004287	0.019084	–0.023371	–0.007515
	174	2.000000	0.067626	0.031716	–0.099342	–0.311342
	175	2.000000	0.098627	0.003907	0.094720	0.028663
HOMO	176	2.000000	0.002422	0.080078	–0.077656	0.001840
LUMO	177	0.000000	0.000000	0.000000	0.000000	0.000000
	178	0.000000	0.000000	0.000000	0.000000	0.000000
sumc		352.000000	0.410403	1.138637	–0.728234	–0.692034

A term is the number of electrons donated from R′–Tl fragment to R′–Sb fragment and the B term is the number of electrons back donated from R′–Sb fragment to R′–Tl fragment. W term is the number of electrons involved in repulsive polarization.

For clearness, only list the A, B, and W terms for HOMO (no. 176) – 10 ∼ LUMO + 2.

Summation of contributions from all unoccupied and occupied orbitals.

Both natural bond orbital (NBO) analysis[77,78] and the natural resonance theory (NRT)[94−96] were applied to determine their electron densities of triple-bonded R′Tl≡SbR′ bearing bulkier ligands, whose calculated results are given in Table . In the case of (SiMe(SitBu3)2)–Tl≡Sb–(SiMe(SitBu3)2), for instance, the Tl≡Sb triple bond is composed of a σ-bond, which is equally distributed on Tl and Sb atoms (Tl: 49%, Sb: 51%). On the other hand, the π component is strongly polarized toward the Sb center (Tl: 15%, Sb: 85% and Tl: 12%, Sb: 88% for π⊥ and π∥, respectively). The strong polarization of the Tl–Sb bond and the high occupancies of its NBO lead to a small WBI of 2.15, all of which suggest the presence of a rather weak Tl≡Sb bond. Further supporting the data obtained by NRT (Table ) demonstrate that the Tl≡Sb triple bond has a short single bond character (25%) and quite a short triple bond character (11%) but a high double bond character (64%) due to the fact the covalent portion of the NRT (1.06) is higher than its ionic part (0.79). In addition, as seen in Table , the dispersion-corrected M06-2X/Def2-TZVP results reveal NBO(Tl≡Sb for π⊥) = 0.385(6s6p99.99)Tl + 0.923(5s5p42.44)Sb and NBO(Tl≡Sb for π∥) = 0.326(6s6p99.99)Tl + 0.944(5s5p6.77)Sb, which strongly demonstrate that the major bonding nature between (SiMe(SitBu3)2)–Tl and (SiMe(SitBu3)2)–Sb components emerges from 6p(Tl) ← 5p(Sb), as schematically shown in Figure S1 (Supporting Information). Accordingly, the above theoretical analyses strongly indicate the triple-bonded R′Tl≡SbR′ compounds featuring bulkier substituents should have a very weak Tl≡Sb triple bond.

Table 3

Natural Bond Orbital and Natural Resonance Theory Analysis for R′Tl≡SbR′ Possessing Four Kinds of Groups (R′ = SiMe(SitBu3)2, SiiPrDis2, Tbt, and Ar*) at the Dispersion-Corrected M06-2X/Def2-TZVP Level of Theorya,b

		NBO analysis			NRT analysis
R′Tl≡SbR′	WBI	occupancy	hybridization	polarization	total/covalent/ionic	resonance weight
R′ = SiMe(SitBu3)2	2.15	σ: 1.75	σ: 0.7004 Tl (sp0.10) + 0.7137 Sb (sp30.96)	49.06% (Tl)	1.86/1.06/0.79	Tl–Sb: 24.88%
				50.94% (Sb)
		π⊥: 1.93	π⊥: 0.3853 Tl (sp99.99) + 0.9228 Sb (sp42.44)	14.85% (Tl)		Tl=Sb: 64.22%
				85.15% (Sb)
		π∥: 1.94	π∥: 0.3256 Tl (sp99.99) + 0.9439 Sb (sp6.77)	11.97% (Tl)		Tl≡Sb: 10.90%
				88.03% (Sb)
R′ = SiiPrDis2	2.21	σ: 1.78	σ: 0.6793 Tl (sp0.87) + 0.7339 Sb (sp21.74)	46.14% (Tl)	2.34/0.85/1.39	Tl–Sb: 24.88%
				53.86% (Sb)
		π⊥: 1.94	π⊥: 0.4034 Tl (sp99.99) + 0.9150 Sb (sp50.48)	16.27% (Tl)		Tl=Sb: 58.68%
				83.73% (Sb)
		π∥: 1.93	π∥: 0.3266 Tl (sp99.99) + 0.9568 Sb (sp88.31)	13.34% (Tl)		Tl≡Sb: 16.44%
				86.66% (Sb)
R′ = Tbt	2.02	σ: 1.98	σ: 0.8111 Tl (sp0.05) + 0.5849 Sb (sp42.92)	65.79% (Tl)	2.04/0.97/1.06	Tl–Sb: 11.79%
				34.21% (Sb)
		π⊥: 1.89	π⊥: 0.3902 Tl (sp91.05) + 0.9604 Sb (sp88.42)	14.27% (Tl)		Tl=Sb: 72.49%
				85.73% (Sb)
		π∥: 1.94	π∥: 0.4145 Tl (sp99.99) + 0.9130 Sb (sp89.92)	17.31% (Tl)		Tl≡Sb: 15.72%
				82.69% (Sb)
R′ = Ar*	2.04	σ: 1.99	σ: 0.8080 Tl (sp0.05) + 0.5891 Sb (sp40.41)	65.29% (Tl)	1.67/0.83/0.85	Tl–Sb: 31.89%
				34.71% (Sb)
		π⊥: 1.89	π⊥: 0.3712 Tl (sp90.97) + 0.9601 Sb (sp89.47)	13.26% (Tl)		Tl=Sb: 68.11%
				86.74% (Sb)
		π∥: 1.94	π∥: 0.4325 Tl (sp99.99) + 0.9151 Sb (sp89.42)	17.31% (Tl)		Tl≡Sb: 0.00%
				82.69% (Sb)

The value of the Wiberg bond index (WBI) for the Tl≡Sb bond and the occupancy of the corresponding σ and π bonding NBO (see refs (77, 78)).

NRT; see refs (94−96).

Overview of the Triple-Bonded RE13≡SbR Molecules

We have used the DFT computations to theoretically design substituted RE13≡SbR molecules that feature an E13≡Sb triple bond, which can be detected and characterized in the experiments. From the comparisons between this work (RTl≡SbR) and other previous studies (RB≡SbR,[72] RAl≡SbR,[73] RGa≡SbR,[74] and RIn≡SbR[75]), several major conclusions can be drawn:

For the triple-bonded RE13≡SbR molecules bearing the small substituents, our theoretical evidences reveal that they should not be observable in the experiments.

Our theoretical observations suggest that only the sterically bulky ligands (R′) can greatly stabilize the triple-bonded R′E13≡SbR′ molecules from the kinetic viewpoint.

Nevertheless, our theoretical results demonstrate that the triple bond between the E13 and antimony elements in the acetylene-like R′E13≡SbR′ species is quite weak. Its bond order is theoretically estimated to be slightly above 2.0.

The reason for having such a weak E13≡Sb triple bond is simple: the atomic sizes as well as the electronegativity values for E13 and Sb are different because they belong to different rows of the periodic table having different principal quantum numbers. This, in turn, makes the overlapping relationship between E13 and antimony small.

On the basis of our theoretical examinations, it is suggested that the bonding nature of the acetylene analogues, R′E13≡SbR′, should be described as R′E13. Namely, its E13≡Sb triple bond contains one conventional σ (E13–Sb) bond, one conventional π (E13=Sb) bond, and one donor–acceptor π (E13 ← Sb) bond.

Our theoretical studies indicate that relativistic effects (or the so-called orbital nonhybridization effect and the inert s-pair effect)[97−100] play a vivid role in determining the geometrical structures of the triple-bonded R′E13≡SbR′ compounds. In other words, the triple-bonded R′E13≡SbR′ molecule possessing sterically bulky groups adopts a bent structure, i.e., ∠R′–E13–Sb < 180.0° and ∠E13–Sb–R′ > 90.0°.

Conclusions

The present study reported on the possibility of the formation of the triple-bonded RTl≡SbR possessing various kinds of chemical substituents. The present theoretical findings demonstrate that only sterically large ligands can greatly stabilize the RTl≡SbR molecule. We anticipate that suppose the introduction of bulky substituents is beneficial, the silyl bulky groups should be a good alternative to bulk-based groups. The computational evidences studied in the present study strongly suggest that the effects of the bulky substituents can definitively increase the bond order between Tl and Sb. This work is the first theoretical study about the possibility of forming of a triple-bonded R′Tl≡SbR′ molecule. It is found that by using the overcrowding large ligands as appropriate substituents (R′), the generation of a triple-bonded R′Tl≡SbR′ molecule is feasible. Nevertheless, similar to the cases of the other triple-bonded RB≡SbR,[72] RAl≡SbR,[73] RGa≡SbR,[74] and RIn≡SbR[75] molecules, our theoretical investigations suggest the bonding natures of such R′Tl≡SbR′ compounds possessing bulkier substituents should be viewed as . In particular, the Tl≡Sb triple bond in R′Tl≡SbR′ possessing sterically bulky ligands is anticipated to be very weak.