Synthesis and Structure of K⁺[<i>ⁱ</i>PrNCP]^-, a 1-Aza-3λ³-phospha-3-allenide^†

2-Isopropyl(trimethylsilyl)amino-1λ3-phosphaalkyne 1 reacts with potassium tert-butoxide to form potassium 1-isopropyl-1-aza-3λ3-phospha-3-allenide (2). This compound was structurally characterized as the corresponding 18-crown-6 ether complex 3. The molecular structure of 1 was also determined in order to compare the bonding situation in the anion and the neutral λ3-phosphaalkyne. Compound 3 contains a nitrogen−carbon−phosphorus group for which the parameters were shown by X-ray structural analysis and quantum chemical calculations to lie between the extrema N−C⋮P and NCP, suggesting reactivity typical of an ambident anion. This is indeed the case, as subsequent reaction of 2 with chlorotrimethylsilane at nitrogen regenerates the λ3-phosphaalkyne 1; with chlorotriphenylsilane the new derivative 4 is formed. In contrast, chlorotrimethylstannane reacts at phosphorus, giving the 1-aza-3λ3-phosphaallene isopropyliminomethylidene(trimethylstannyl)phosphane 5.