Synthesis and properties of [NiCp*(2,5-tBu2PC4H2)], a 20-valence-electron phosphanickelocene.

The reaction of the bulky phospholide salt Li(2,5- tBu2PC4H2)⋅2 THF (1; THF=tetrahydrofuran) with [NiCp*(acac)] (HCp*=pentamethylcyclopentadiene, Hacac=acetylacetone) gives the green air-sensitive phosphanickelocene [NiCp*(2,5-tBu2PC4H2)] (2) in yields of about 85 %. An X-ray structural determination of 2 shows long Niring bonds and “delocalised” ring PC and CC bonds characteristic of a classical 20-valence-electron (ve) nickelocene. The electronic structure of 2 has been clarified through a combined Amsterdam density functional (ADF) and photoelectron spectroscopic study, which indicates that the higher lying semi-occupied molecular orbital (SOMO) (−5.82 eV) has a′ symmetry and that the phosphorus “lone pair” is energetically low-lying (−8.15 eV). Oxidation of phosphanickelocene 2 by AgBF4 occurs quantitatively to give the corresponding air-sensitive orange phosphanickelocenium salt [NiCp*(2,5-tBu2PC4H2)][BF4] (3). This complex has also been characterised by an X-ray crystallographic study, which reveals long NiCα and short CαCβ bonds in the phospholyl ligand indicative of a SOMO having a′′ symmetry. PMe3 reacts with 2 at room temperature to provoke a ring-slip reaction that gives the 18ve complex [NiCp*η1-(2,5-tBu2PC4H2)(PMe3)] (4), but shows no reaction with the phosphanickelocenium salt 3 under the same conditions.