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

2-Isopropyl(trimethylsilyl)amino-1<i>λ</i><sup>3</sup>-phosphaalkyne <b>1</b> reacts with potassium <i>tert</i>-butoxide to form potassium 1-isopropyl-1-aza-3<i>λ</i><sup>3</sup>-phospha-3-allenide (<b>2</b>). This compound was structurally characterized as the corresponding 18-crown-6 ether complex <b>3</b>. The molecular structure of <b>1</b> was also determined in order to compare the bonding situation in the anion and the neutral <i>λ</i><sup>3</sup>-phosphaalkyne. Compound <b>3</b> 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 <b>2</b> with chlorotrimethylsilane at nitrogen regenerates the <i>λ</i><sup>3</sup>-phosphaalkyne <b>1</b>; with chlorotriphenylsilane the new derivative<b> 4</b> is formed. In contrast, chlorotrimethylstannane reacts at phosphorus, giving the 1-aza-3<i>λ</i><sup>3</sup>-phosphaallene isopropyliminomethylidene(trimethylstannyl)phosphane <b>5</b>.