A Digermanium(III) 1,2-Dication Stabilized by Amidinate and cAAC-Phosphinidenide Ligands.

A digermanium(III) 1,2-dication comprises two cationic centers located at two interconnected Ge atoms. The strong Coulombic repulsion between two positively charged germanium cations hinders their bond formation. Balancing these two oppositions was achieved by using amidinate and cyclic (alkyl)amino carbene (cAAC)-phosphinidenide ligands, where an amidinato cAAC-phosphinidenidogermylene complex, [LGeP(cAACMe)] (2, where L = PhC(NtBu)2, cAACMe = :C{C(Me)2CH2C(Me)2NAr}, and Ar = 2,6-iPr2C6H3), underwent one-electron oxidation with a bis(phosphinidene) radical cation, [(cAACMe)P]2•+, to form a digermanium(III) 1,2-dication, [LGeP(cAACMe)]22+, in compound 4.

N-Phosphinoamidinato Silylene- and Phosphine-Borylborylene Complexes.

This work describes a straightforward method to synthesize a borylborylene without proceeding via the rearrangement of a diborene. An amidinato amidosilylene [LSiNMe2] (L = PhC(NtBu)2) and PMe3 were reacted with an N-phosphinoamidinato diborane 1 and KC8 to form a stable silylene-borylborylene 2 and a persistent phosphine-borylborylene 3, respectively. Compound 2 is stable as the borylene center is well stabilized by the silylene donor and boryl substituent, whereas compound 3 is unstable in solution due to labile PMe3. The latter was illustrated by reacting compound 3 with Ar'NC (Ar' = 2,6-Me2C6H3), where Ar'NC displaced PMe3 and inserted into the N-phosphinoamidinate ligand and B-B bond to form compound 4.

A Pyridine-Stabilized N-Phosphinoamidinato N-Heterocyclic Carbene-Diboravinyl Cation: Boron Analogue of Vinyl Cation.

A boron analogue of vinyl cation, pyridine-stabilized N-phosphinoamidinato N-heterocyclic carbene (NHC)-diboravinyl cation 2+ , was synthesized by displacement of bromide in diborene 1 with excess pyridine. Experimental and computational studies showed that the positive charge is mainly at the B-B skeleton with delocalization to the pyridine ligand. One of the main modes of reactivity is through the B=B double bond alongside activation of the pyridine substituent, where the Bpyridine center is the predominant nucleophilic center and the predominant electrophilic center is either the activated pyridine para position or the BNHC center, illustrating the presence of diborene cation A, borylene-borenium cation B and diborene-pyridinium cation C resonance structures in cation 2+ .

Understanding the Reactivity of Combination Reactions of Intramolecular Geminal Group 13 Element/Phosphorus and Gallium/Group 15 Element Frustrated Lewis Pairs with CS2.

The reactions of CS2 captured by intramolecular geminal G13/P-based (G13 = group 13 elements) and Ga/G15-based (G15 = group 15 elements) frustrated Lewis pairs have been theoretically examined by using density functional theory (DFT) computations. With regard to the nine FLP-related compounds, our DFT calculated results reveal that only Al/P-Rea and Ga/P-Rea can kinetically and thermodynamically precede the energetically feasible combination reactions with CS2 to form the five-membered heterocyclic adducts. Our activation strain model analyses on the nine aforementioned model molecules indicate that the atomic radius of the Lewis acceptor (G13) and the Lewis donor (G15) plays a role in controlling their barrier heights to obtain good orbital overlaps among G13/P-Rea, Ga/G15-Rea, and CS2. Our theoretical observations based on the energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) approach strongly indicate that the donor-acceptor bonding (i.e., singlet-singlet bonding) rather than the electron-sharing bonding (i.e., triplet-triplet bonding) plays a central role in determining the bonding conditions of the transition states, G13/P-TS and Ga/G15-TS. In addition, the theoretical evidence obtained by the frontier molecular orbital theory and EDA-NOCV analyses reveals that the best description for the bonding natures of the combination reactions of intramolecular geminal G13/P-Rea and Ga/G15-Rea with CS2 is the lone pair(G15) → p-π*(C) interaction rather than the p-π*(G13) ← p-π(S) interaction. Moreover, our present DFT computations concerning the calculated structures and corresponding relative energetics of the stationary points connected with the aforementioned sophisticated approaches are in accordance with the Hammond postulate.

Reversible CO2 activation by a N-phosphinoamidinato digermyne.

The N-phosphinoamidinato digermynes [LG̈e-G̈eL] (L = tBu2PNC(Ph)NAr, 4: Ar = 2,6-iPr2C6H3, 5: Ar = Ph) underwent reversible CO2 activation to form [LG̈eOC(O)G̈eL] (6: Ar = 2,6-iPr2C6H3, 7: Ar = Ph). Compound 7 was further reacted with diphenylacetylene and hexafluorobenzene, which proceeded through compound 5 in the first step, to form CO2, [LG̈eC(Ph) = C(Ph) G̈eL] (8), [LG̈eF] (9) and [LG̈eC6F5] (10), respectively.

Amidinatoamidosilylene-Dibromodiborene.

The amidinatoamidosilylene [LSiNMe2] [1; L = PhC(NtBu)2] was reacted with B2Br4(SMe2)2 in toluene at room temperature to form the bis(silylene)tetrabromodiborane [L{Me2N}Si]2B2Br4 (2). It was then reacted with excess KC8 in tetrahydrofuran at room temperature to afford the bis(silylene)dibromodiborene [L{Me2N}Si]2B2Br2 (3).

Significant Insight into the Origin of Reaction Barriers Determining Dihydrogen Activation by G13-P-P (G13 = Group 13 Element) and G15-P-Ga (G15 = Group 15 Element) Frustrated Lewis Pairs.

The heterolytic cleavage of H2 by multiply bonded phosphorus-bridged G13-P-P-Rea (G13 = B, Al, Ga, In, and Tl) and G15-P-Ga-Rea (G15 = N, P, As, Sb, and Bi) frustrated Lewis pairs (FLPs) has been theoretically investigated using density functional theory calculations. For the above nine FLP-type molecules, our theoretical findings suggest that only Al-P-P-Rea, Ga-P-P-Rea, and In-P-P-Rea can undergo the energetically feasible H2 activation reaction from kinetic and thermodynamic viewpoints. Our study based on the activation strain model (ASM) reveals that gaining a better orbital overlap between G13-P-P-Rea and G15-P-Ga-Rea molecules and H2 affected the reaction barriers through the atomic radius of G13 and G15. According to our energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) results, the bonding of these H2 activation reactions involving G13-P-P-Rea and G15-P-Ga-Rea is dominated by the donor-acceptor interaction (singlet-singlet interaction) rather than the electron-sharing interaction (triplet-triplet interaction). Moreover, our EDA-NOCV evidence reveals that the best description for the above bonding situations is the lone pair(G15) → σ*(H2) interaction rather than the empty p-π-orbital(G13) ← σ(H2) interaction. In particular, the findings in this work based on theoretically calculated geometries and the corresponding relative free energies of the stationary points combined with the results from the above sophisticated methods nicely agree with the famous Hammond postulate.

A N-Phosphinoamidinato NHC-Diborene Catalyst for Hydroboration.

The use of the N-phosphinoamidinato NHC-diborene catalyst 2 for hydroboration is described. The N-phosphinoamidine tBu2PN(H)C(Ph)═N(2,6-iPr2C6H3) was reacted with nBuLi in Et2O to afford the lithium derivative, which was then treated with B2Br4(SMe2)2 in toluene to form the N-phosphinoamidinate-bridged diborane 1. It was reacted with the N-heterocyclic carbene IMe (:C{N(CH3)C(CH3)}2) and excess potassium graphite at room temperature in toluene to give the N-phosphinoamidinato NHC-diborene compound 2. It can stoichiometrically activate ammonia-borane and carbon dioxide. It also showed catalytic capability. A 2 mol % portion of 2 catalyzed the hydroboration of carbon dioxide (CO2) with pinacolborane (HBpin) in deuterated benzene (C6D6) at 110 °C (conversion >99%), which afforded the methoxyborane [pinBOMe] (yield 97.8%, TOF 33.3 h-1) and the bis(boryl) oxide [(pinB)2O]. In addition, 5 mol % of 2 catalyzed the N-formylation of secondary and primary amines by carbon dioxide and pinacolborane to yield the N-formamides (average yield 91.6%, TOF 25.9 h-1). Moreover, 2 showed chemoselectivity toward catalytic hydroboration of carbonyl compounds. In mechanistic studies, the B═B double bond in compound 2 activated the substrates, the intermediates of which then underwent hydroboration with pinacolborane to yield the products and regenerate catalyst 2.

A theoretical study of the reactivity of ethene and benzophenone with a hyper-coordinated alkene containing a so-called E=E (E = C, Si, Ge, Sn, and Pb) unit.

The reactivity of a reported hyper-coordinated alkene (Rea-E; Rea = reactant; E = group 14 element) featuring a central E[double bond, length as m-dash]E moiety was theoretically analyzed using DFT (density functional theory) and the EDA-NOCV (energy decomposition analysis-natural orbitals for chemical valence) method. M06-2X/def2-SVP and B3LYP-D3/def2-SVP results demonstrate that five Rea-E molecules have an energy minimum as their structures have no imaginary frequency. Theoretical examinations based on three types of bond order calculations (Wiberg, Mayer, and Fuzzy), the LOL (localized orbital locator) analyses, Lewis structures and the NBO (natural bond orbital) analyses suggest that a very weak central Si-Si single bond and an extremely weak central Ge-Ge single bond, rather than a double bond, are present in the Rea-Si and Rea-Ge molecules, respectively. On the other hand, no bond is found between the two central group 14 atoms in Rea-C, Rea-Sn, and Rea-Pb. The theoretical investigation demonstrates that the reactivity of the Rea-E compound decreases in the order Rea-Si > Rea-Ge > Rea-C, a trend that results from the differences in the atomic radii of the group 14 elements. Carbon has the smallest atomic radius in the group 14 family, causing steric crowding between Rea-C and other attacking species. This circumstance, in turn, increases the activation energies of its addition reactions and renders these reactions energetically infeasible. For the cyclic product of Rea-Ge, the theoretical evidence reveals that the comparatively large atomic radius of Ge induces the weakest Pauli repulsions and the smallest overlap integrals between Rea-Ge and the other doubly bonded molecules. This situation, in turn, makes the overall cyclization reaction of Rea-Ge endothermic. As a result, only the silicon-centered molecule, Rea-Si, can undergo the [2 + 2] cycloaddition reactions with doubly bonded molecules without kinetic or thermodynamic difficulty, which agrees well with the available experimental findings.

A Versatile NHC-Parent Silyliumylidene Cation for Catalytic Chemo- and Regioselective Hydroboration.

This study describes the first use of a silicon(II) complex, NHC-parent silyliumylidene cation complex [(IMe)2SiH]I (1, IMe = :C{N(Me)C(Me)}2) as a versatile catalyst in organic synthesis. Complex 1 (loading: 10 mol %) was shown to act as an efficient catalyst (reaction time: 0.08 h, yield: 94%, TOF = 113.2 h-1; reaction time: 0.17 h, yield: 98%, TOF = 58.7 h-1) for the selective reduction of CO2 with pinacolborane (HBpin) to form the primarily reduced formoxyborane [pinBOC(═O)H]. The activity is better than the currently available base-metal catalysts used for this reaction. It also catalyzed the chemo- and regioselective hydroboration of carbonyl compounds and pyridine derivatives to form borate esters and N-boryl-1,4-dihydropyridine derivatives with quantitative conversions, respectively. Mechanistic studies show that the silicon(II) center in complex 1 activated the substrates and then mediated the catalytic hydroboration. In addition, complex 1 was slightly converted into the NHC-borylsilyliumylidene complex [(IMe)2SiBpin]I (3) in the catalysis, which was also able to mediate the catalytic hydroboration.

A Theoretical Study on the Stability of PtL2 Complexes of Endohedral Fullerenes: The Influence of Encapsulated Ions, Cage Sizes, and Ligands.

The {η2-(X@C n )}PtL2 complexes possessing three kinds of encapsulated ions (X = F-, Ø, Li+), three various ligands (L = CO, PPh3, NHCMe), and twelve cage sizes (C60, C70, C72, C74, C76, C78, C80, C84, C86, C90, C96, C100) are theoretically examined by using the density functional theory (M06/LANL2DZ). The present computational results demonstrate that the backward-bonding orbital interactions, rather than the forward-bonding orbital interactions, play a dominant role in the stability of {η2-(X@C n )}PtL2 complexes. Additionally, our theoretical study shows that the presence of the encapsulated Li+ ion can greatly improve the stability of {η2-(X@C n )}PtL2 complexes, whereas the existence of the encapsulated F- ion can heavily reduce the stability of {η2-(X@C n )}PtL2 complexes. Moreover, the theoretical evidence strongly suggests that the backward-bonding orbital interactions as well as the stability increase in the order {η2-(X@C n )}Pt(CO)2 < {η2-(X@C n )}Pt(PPh3)2 < {η2-(X@C n )}Pt(NHCMe)2. As a result, these theoretical observations can provide experimental chemists a promising synthetic direction.

A self-hydrosilylation of phosphanylhydrosilylalkynes promoted by B(C6F5)3? An experimental and mechanistic study.

Phosphanylhydrosilylalkynes Me2HSiC[triple bond, length as m-dash]CPAr2 (Ar = Ph, 1a; 4-MeC6H4, 1b) were synthesized, which reacted with B(C6F5)3 to produce alkenes [(E)-(C6F5)3BCH[double bond, length as m-dash]C(PAr2)SiMe2]2 (2a and 2b) and (Z)-(C6F5)2BCH[double bond, length as m-dash]C(PAr2)SiMe2(C6F5) (3a and 3b). The formation of 2a (or 2b) involved a Wrackmeyer's SiHMe2 migration followed by Si-H addition across the C[triple bond, length as m-dash]C bond, whereas, that of 3a (or 3b) involved a similar mechanism with a further C6F5 migration. The B(C6F5)3-promoted reaction of the Si-centered geminal H and C[triple bond, length as m-dash]C groups is thus realized, which may be considered as a self-hydrosilylation. Mechanistic studies by both variable temperature NMR spectroscopy and DFT calculations were accomplished.

A computational study to determine whether substituents make E13[triple bond, length as m-dash]nitrogen (E13 = B, Al, Ga, In, and Tl) triple bonds synthetically accessible.

This study theoretically determines the effect of substituents on the stability of the triple-bonded L-E13[triple bond, length as m-dash]N-L (E13 = B, Al, Ga, In, and Tl) compound using the M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, and B3LYP/LANL2DZ+dp levels of theory. Five small substituents (F, OH, H, CH3 and SiH3) and four large substituents (SiMe(SitBu3)2, SiiPrDis2, Tbt ([double bond, length as m-dash] C6H2-2,4,6-{CH(SiMe3)2}3) and Ar* ([double bond, length as m-dash]C6H3-2,6-(C6H2-2,4,6-i-Pr3)2)) are used. Unlike other triply bonded L-E13[triple bond, length as m-dash]P-L, L-E13[triple bond, length as m-dash]As-L, L-E13[triple bond, length as m-dash]Sb-L and L-E13[triple bond, length as m-dash]Bi-L molecules that have been studied, the theoretical findings for this study show that both small (but electropositive) ligands and bulky substituents can effectively stabilize the central E13[triple bond, length as m-dash]N triple bond. Nevertheless, these theoretical observations using the natural bond orbital and the natural resonance theory show that the central E13[triple bond, length as m-dash]N triple bond in these acetylene analogues must be weak, since these E13[triple bond, length as m-dash]N compounds with various ligands do not have a real triple bond.

Synthesis of a Dimeric Base-Stabilized Cobaltosilylene Complex for Catalytic C-H Bond Functionalization and C-C Bond Formation.

The synthesis of a dimeric base-stabilized cobaltosilylene complex and its catalytic reactions are described. Treatment of the amidinato silicon(I) dimer [LSi:]2 (1; L=PhC(NtBu)2 ) with CoBr2 in toluene for 10 days afforded the dimeric amidinato cobaltosilylene [(LSi)µ-{CoBr(LSiBr)}]2 (2), which is speculated to proceed via "LSiCoBr" and "LSiBr" intermediates in the reaction. Compound 2 is paramagnetic, with an effective magnetic moment of 2.8 µB. Its electronic structure was elucidated by single-crystal X-ray crystallography and DFT studies. It was capable of catalyzing C-H bond functionalization, in which a combination of 2, phosphine and MeMgI can regio- and stereoselectively promoted the addition of the C≡C triple bonds in alkynes to the ortho-C-H position in arylpyridines. In addition, compound 2 catalyzed Kumada-type coupling reactions between aryl chlorides and the Grignard reagent 2-mesitylmagnesium bromide.

B-H Bond Activation by an Amidinate-Stabilized Amidosilylene: Non-Innocent Amidinate Ligand.

The activation of B-H and B-Cl bonds in boranes by base-stabilized low-valent silicon compounds is described. The reaction of the amidinato amidosilylene-borane adduct [L{Ar(Me3Si)N}SiBH3] [1; L = PhC(N tBu)2, and Ar = 2,6- iPr2C6H3] with MeOTf in toluene at room temperature formed [L{Ar(Me3Si)N}SiBH2OTf] (2). [LSiN(SiMe3)Ar] in compound 2 then underwent a B-H bond activation with BH2OTf in refluxing toluene to afford the B-H bond activation product [LB(H)Si(H)(OTf){N(SiMe3)Ar}] (3). On the other hand, when compound 2 was reacted with 4-dimethylaminopyridine in refluxing toluene, another B-H bond activation product [(µ-κ1:κ1-L)B(H)(DMAP)Si(H){N(Ar)SiMe3}]OTf (4) was afforded. Mechanistic studies show that "(µ-κ1:κ1-L)B(H)(OTf)Si(H){N(Ar)SiMe3}" (2A) is the key intermediate in the reactions mentioned above. The formation of 2A is further evidenced by the activation of the B-Cl bond in PhBCl2 by the amidinato silicon(I) dimer [LSi:]2 to form the B-Cl bond activation product [(µ-κ1:κ1-L)B(Cl)(Ph)Si(Cl)]2 (6). Compounds 2-4 and 6 were characterized by nuclear magnetic resonance spectroscopy and X-ray crystallography.

Triple-Bonded Boron≡Phosphorus Molecule: Is That Possible?

The effect of substitution on the potential energy surfaces of RB≡PR (R = H, F, OH, SiH3, and CH3) is studied using density functional theories (M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, and B3LYP/LANL2DZ+dp). There is significant theoretical evidence that RB≡PR compounds with smaller substituents are fleeting intermediates, so they would be difficult to be detected experimentally. These theoretical studies using the M06-2X/Def2-TZVP method demonstrate that only the triply bonded R'B≡PR' molecules bearing sterically bulky groups (R' = Tbt (=C6H2-2,4,6-{CH(SiMe3)2}3), SiMe(SitBu3)2, Ar* (=C6H3-2,6-(C6H2-2,4,6-i-Pr3)2), and SiiPrDis2) are significantly stabilized and can be isolated experimentally. Using the simple valence-electron bonding model and some sophisticated theories, the bonding character of R'B≡PR' should be viewed as R'BI PR'. The present theoretical observations indicate that both the electronic and the steric effect of bulkier substituent ligands play a key role in making triply bonded R'B≡PR' species synthetically accessible and isolable in a stable form.

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.

Triply Bonded Gallium≡Phosphorus Molecules: Theoretical Designs and Characterization.

The effect of substitution on the potential energy surfaces of triple-bonded RGa≡PR (R = F, OH, H, CH3, SiH3, SiMe(SitBu3)2, SiiPrDis2, Tbt (C6H2-2,4,6-{CH(SiMe3)2}3), and Ar* (C6H3-2,6-(C6H2-2,4,6-i-Pr3)2)) compounds was theoretically examined by using density functional theory (i.e., M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, and B3LYP/LANL2DZ+dp). The theoretical evidence strongly suggests that all of the triple-bonded RGa≡PR species prefer to select a bent form with an angle (∠Ga-P-R) of about 90°. Moreover, the theoretical observations indicate that only the bulkier substituents, in particular, for the strong donating groups (e.g., SiMe(SitBu3)2 and SiiPrDis2) can efficiently stabilize the Ga≡P triple bond. In addition, the bonding analyses (based on the natural bond orbital, the natural resonance theory, and the charge decomposition analysis) reveal that the bonding characters of such triple-bonded RGa≡PR molecules should be regarded as [Formula: see text]. In other words, the Ga≡P triple bond involves one traditional σ bond, one traditional π bond, and one donor-acceptor π bond. Accordingly, the theoretical conclusions strongly suggest that the Ga≡P triple bond in such acetylene analogues (RGa≡PR) should be very weak.

A Dimeric NHC-Silicon Monotelluride: Synthesis, Isomerization, and Reactivity.

The reaction of the NHC-disilicon(0) complex [(IAr )Si=Si(IAr )] (1, IAr =:C{N(Ar)C(H)}2 , Ar=2,6-iPr2 C6 H3 ) with two equiv of elemental Te in toluene at room temperature for three days afforded a mixture of the first dimeric NHC-silicon monotelluride [(IAr )Si=Te]2 (2) and its isomeric complex [(IAr )Si(µ-Te)Si(IAr )=Te] (3). When the same reaction was performed for ten days, only 3 was isolated from the reaction mixture. Compound 1 reacted with four equiv of elemental Te in toluene for four weeks, which proceeded through the formation of 2, 3 and the NHC-disilicon tritelluride complex [{(IAr )Si(=Te)}2 Te] (5-Te), to form the dimeric NHC-silicon ditelluride [(IAr )Si(=Te)(µ-Te)]2 (4). The reactions are in line with theoretical mechanistic studies for the formation of 4. Compound 3 reacted with one equiv of elemental sulfur in toluene to form the first NHC-disilicon sulfur ditelluride complex [{(IAr )Si(=Te)}2 S] (5-S).

Substituent Effects on the Stability of Thallium and Phosphorus Triple Bonds: A Density Functional Study.

Three computational methods (M06-2X/Def2-TZVP, B3PW91/Def2-TZVP and B3LYP/LANL2DZ+dp) were used to study the effect of substitution on the potential energy surfaces of RTl≡PR (R = F, OH, H, CH3, SiH3, SiMe(SitBu3)2, SiiPrDis2, Tbt (=C6H2-2,4,6-(CH(SiMe3)2)3), and Ar* (=C6H3-2,6-(C6H2-2, 4,6-i-Pr3)2)). The theoretical results show that these triply bonded RTl≡PR compounds have a preference for a bent geometry (i.e., ∠R⎼Tl⎼P ≈ 180° and ∠Tl⎼P⎼R ≈ 120°). Two valence bond models are used to interpret the bonding character of the Tl≡P triple bond. One is model [I], which is best described as TlP. This interprets the bonding conditions for RTl≡PR molecules that feature small ligands. The other is model [II], which is best represented as TlP. This explains the bonding character of RTl≡PR molecules that feature large substituents. Irrespective of the types of substituents used for the RTl≡PR species, the theoretical investigations (based on the natural bond orbital, the natural resonance theory, and the charge decomposition analysis) demonstrate that their Tl≡P triple bonds are very weak. However, the theoretical results predict that only bulkier substituents greatly stabilize the triply bonded RTl≡PR species, from the kinetic viewpoint.