A Crystalline Monomeric Phosphaborene.

We report the synthesis of the monomeric phosphaborene Ar*P═B(TMP) (2) (Ar* = 2,6-bis(triisopropylphenyl)-3,5-diisopropylphenyl) containing 2-coordinate phosphorus and boron centers. Compound 2 has a PB bond length of 1.741(3) Å, the shortest reported to date. Computational examination of the bonding in 2 reveals, in addition to the σ bond, the presence of a single classical π bond and a large Wiberg bond index of 1.9707, consistent with double bond, and not triple bond, character. The chemistry of 2 is marked by its low reactivity, which is rationalized by examination of the frontier molecular orbitals and steric considerations.