Aluminum–phosphorus triple bonds: Do substituents make Al[tbnd6]P synthetically accessible?

Abstract

The effect of substitution on the potential energy surfaces of triple-bonded RAlPR (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 investigated by using the theoretical methods (i.e., M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, B3LYP/LANL2DZ+dp, and CCSD(T)). The theoretical examinations reveal that all of the triple-bonded RAlPR species prefer to adopt a bent form with a roughly perpendicular bond angle (∠Al–P–R). In addition, the theoretical evidence demonstrates that only the bulkier substituents can efficiently stabilize the central AlP triple bond. Moreover, the theoretical analyses (the natural bond orbital, the natural resonance theory, and the charge decomposition analysis) indicate that the bonding characters of the triply bonded RAlPR molecules should be described as R′Al▪PR′. That is to say, the AlP triple bond contains one conventional σ bond, one conventional π bond, and one donor-acceptor π bond. Nevertheless, the theoretical conclusions based on the poor overlap populations between Al and P elements suggest the AlP triple bond in such an acetylene analogues (RAlPR) is likely to be very weak.