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

Salt metathesis of dinickel(II) complex LNiBr (1; L is a dinucleating pyrazolate ligand with two β-diketiminato chelate arms) with Na(OCP) ⋅ (dioxane) yielded LNi(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 LNi(PCN-CHPh) (3) and LNiP(NHC) (4) with P-bridging cyanophosphide and NHC-phosphinidenide, respectively. Electronic structure analysis indicated a μ-η : η binding mode of the PCO anion between the two Ni ions in 2, which is even more pronounced for the [PCN(-CHPh)] 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 S2 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 KPPh, giving unstable K[LNi(P(O)CPPh)] (5) with a P-bridging and K-stabilized diphosphaurea derivative. All new complexes 2-5 have been comprehensively characterized, including by X-ray diffraction.