Synthesis and properties of [NiCp*(2,5-tBu₂PC ₄H₂)], a 20-valence-electron phosphanickelocene

The reaction of the bulky phospholide salt Li(2,5-tBu₂PC ₄H₂)·2 THF (1; THF = tetrahydrofuran) with [NiCp*(acac)] (HCp* = pentamethylcyclopentadiene, Hacac = acetylacetone) gives the green air-sensitive phosphanickelocene [NiCp*(2,5-trBu₂PC₄H₂)] (2) in yields of about 85%. An X-ray structural determination of 2 shows long Ni-ring bonds and "delocalised" ring P-C and C-C bonds characteristic of a classical 20-valenceelectron (ve) nickelocene. The electronic structure of 2 has been clarified through a combined Amsterdam densi ty 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 correspond ing 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 NiC_α and short C_α- C_β 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.