The effect of substitution on the potential energy surfaces of RB≡PR (R = H, F, OH, SiH 3 , and CH 3 ) 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 (=C 6 H 2 -2,4,6-{CH(SiMe 3 ) 2 } 3 ), SiMe(Si t Bu 3 ) 2 , Ar* (=C 6 H 3 -2,6-(C 6 H 2 -2,4,6- i -Pr 3 ) 2 ), and Si i PrDis 2 ) 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.

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The effect of substitution on the potential energy surfaces of triple-bonded RGa≡PR (R = F, OH, H, CH, SiH, SiMe(SitBu), SiiPrDis, Tbt (CH-2,4,6-{CH(SiMe)}), and Ar* (CH-2,6-(CH-2,4,6-i-Pr))) 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(SitBu) and SiiPrDis) 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.

The substituent effects on the potential energy surfaces of RBBiR (R = F, OH, H, CH3, SiH3, Tbt, Ar*, SiMe­(SitBu3)2, and SiiPrDis2) are determined using density functional theories (M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, and B3LYP/LANL2DZ+dp). The theoretical results show that all of the triply bonded RBBiR molecules prefer to adopt a bent geometry (i.e., ∠RBBi ≈ 180° and ∠BBiR ≈ 90°), which agrees well with the valence-electron bonding model. It is also demonstrated that the smaller groups, such as R = H, F, OH, CH3, and SiH3, neither kinetically nor thermodynamically stabilize the triply bonded RBBiR compounds, except for H3SiBBiSiH3. However, triply bonded R′BBiR′ molecules that feature bulkier substituents (R′ = SiiPrDis2, SiMe­(SitBu3)2, Tbt, and Ar*) are predicted to have a thermodynamic and kinetic global minimum. This theoretical study finds that both the steric and the electronic effects of bulkier substituent groups play a significant role in forming triply bonded RBBiR species that are experimentally obtainable and isolable in a stable form.

The effect of substitution on the potential energy surfaces of RGa[triple bond, length as m-dash]SbR (R = F, OH, H, CH, SiH, SiMe(SitBu), SiiPrDis and NHC) is studied using density functional theory (M06-2X/Def2-TZVP, B3PW91/Def2-TZVP and B3LYP/LANL2DZ + dp). The computational results show that all of the triply bonded RGa[triple bond, length as m-dash]SbR molecules have a preference for a bent geometry (i.e., ∠RGaSb ≈ 180° and ∠GaSbR ≈ 90°), which can be described using a valence bond model. The theoretical results show that because RGa[triple bond, length as m-dash]SbR has smaller electropositive groups, it could be both kinetically and thermodynamically stable and may be experimentally detectable. However, these theoretical results predict that the triply bonded R'Ga[triple bond, length as m-dash]SbR' molecules that have bulkier groups (R' = SiMe(SitBu), SiiPrDis, and NHC) are kinetically stable. In other words, both the electronic and the steric effects of bulkier substituent groups mean that it should be possible to experimentally isolate triply bonded RGa[triple bond, length as m-dash]SbR molecules in a stable form.

Three (M06-2X/Def2-TZVP, B3PW91/Def2-TZVP and B3LYP/LANL2DZ+dp) levels of theory are used to study the effect of substituents on the potential energy surfaces of RB[triple bond, length as m-dash]SbR (R = F, OH, H, CH, SiH, SiMe(SitBu), SiiPrDis and NHC). The theoretical results demonstrate that the triply bonded RB[triple bond, length as m-dash]SbR molecules favor a bent geometry: that is, ∠R-B-Sb ≈ 180° and ∠B-Sb-R ≈ 120°. Regardless of the type of substituents that are attached to the RB[triple bond, length as m-dash]SbR compounds, theoretical evidence strongly indicates that their B[triple bond, length as m-dash]Sb triple bonds have a donor-acceptor nature and are proven to be very weak. Two valence bond models clarify the bonding characters of the B[triple bond, length as m-dash]Sb triple bond. For RB[triple bond, length as m-dash]SbR molecules that feature small substituents, the triple bond is represented as . For RB[triple bond, length as m-dash]SbR molecules that feature large substituents, the triple bond is represented as . Most importantly, this theoretical study predicts that only bulkier substituents significantly stabilize the triply bonded RB[triple bond, length as m-dash]SbR molecules, from the kinetic viewpoint.

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.

Only bulkier substituents can thermodynamically stabilize the triple-bonded RInSbR molecules.