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, CH(3), SiH(3), SiMe(SitBu(3))(2), SiiPrDis(2), Tbt (=C(6)H(2)-2,4,6-(CH(SiMe(3))(2))(3)), and Ar* (=C(6)H(3)-2,6-(C(6)H(2)-2, 4,6-i-Pr(3))(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 Tl [Image: see text] P. This interprets the bonding conditions for RTl≡PR molecules that feature small ligands. The other is model [II], which is best represented as Tl [Image: see text] P. 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.