The rigidity of the PNP ligand enforces the coordination of the phosphine arms to the Ta center and promotes double α-abstraction in (PNP)TaMe4 to yield the novel bis(methylidene) complex (PNP)Ta(CH2)2. The latter is thermally stable and does not appear to contain agostic interactions with the methylidene ligands.
Reaction of (PNP)Li with TaF5 produces pentagonal-bipyramidal (PNP)TaF4 (2). Alkylation of 2 with MeMgBrallows for the isolation of (PNP)TaMe4 (3). (PNP)TaMe4 (3)evolves thermally and/or photochemically into a bis(methylidene) complex (PNP)Ta(CH2)2 (4). The identity of the latterhas been established by X-ray structural, NMR spectroscopic,and DFT computational studies. It does not appear that 4possesses agostic interactions in solution.