Structural studies by electron diffraction reveal that phosphorine is a planar
aromatic compound with 88% of aromaticity of that of
benzene. Potentially relevant to its high aromaticity are the well matched
electronegativities of phosphorus (2.1) and
carbon (2.5). The P–C
bond length is 173
pm and the C–C bond lengths center around 140 pm and show little variation. Although phosphorine and
pyridine are structurally similar, phosphorines are far less basic. The p
Ka of C5H5PH+ and C5H5NH+ are respectively −16.1 and +5.2. The P-oxides are extremely unstable, rapidly adding nucleophiles to a species tetracoordinate at phosphorus. Strongly backbonding Lewis acids (e.g.
tungsten pentacarbonyl) can stabilize a
dative bond from phosphorus. Both
electrophiles and strong, hard
nucleophiles preferentially attack at phosphorus, but the ring aromaticity is sufficiently weak that the result is an
addition reaction, and not aromatic substitution. Halophosphorines do undergo noble-metal- or
zirconocene-catalyzed substitution, and λ5-phosphorines exhibit
a much more traditional substitution chemistry. Unlike
arsabenzene, phosphorine rarely participates in
Diels-Alder-type cycloadditions; when it does, the coupling partner must be an extremely electron-poor alkyne. Phosphorine
complexes are tolerable Diels-Alder reactants.
Coordination chemistry Coordination complexes bearing phosphorine as a
ligand are known. Phosphorines can bind to metals through phosphorus center. Complexes of the diphospha analogue of
2,2′-bipyridine are known. Phosphorines also form pi-complexes, illustrated by V(
η6-C5H5P)2. == See also ==