From a P-Bridging Phosphaketene to μ-Phosphinidenide and μ-Diphosphaurea Units at a Dinickel Core

Salt metathesis of dinickel(II) complex LNi₂Br (1; L is a dinucleating pyrazolate ligand with two β-diketiminato chelate arms) with Na(OCP) ⋅ (dioxane)_2.5 yielded LNi₂(PCO) (2) with a P-bridging phosphaethynolate. Further reaction of 2 with benzyl isocyanide or with an N-heterocyclic carbene (NHC) triggered decarbonylation and gave LNi₂(PCN-CH₂Ph) (3) and LNi₂P(NHC) (4) with P-bridging cyanophosphide and NHC-phosphinidenide, respectively. Electronic structure analysis indicated a μ₂-η² : η¹ binding mode of the PCO^- anion between the two Ni^II ions in 2, which is even more pronounced for the [PCN(-CH₂Ph)]^- anion in 3. DFT assessment of the formation mechanism of 4 showed that attack at the phosphaketene-C atom is kinetically preferred but reversible and unproductive, while kinetically more demanding back-side S_N2 attack at the phosphaketene-P atom triggers CO release with 4 as thermodynamic product. Nucleophilic addition at the phosphaketene-C could be demonstrated by the strongly exergonic reaction of 2 with KPPh₂, giving unstable K[LNi₂(P(O)CPPh₂)] (5) with a P-bridging and K⁺-stabilized diphosphaurea derivative. All new complexes 2-5 have been comprehensively characterized, including by X-ray diffraction.