The synthesis of trans-1,2-dihydro-1-(2,4,6-tri-tert-butylphenyl)-2-(dicarbonyl-η5-pentamethylcyclopentadienylferrio)-4-diisopropylamino-1,2,3-triphosphete (9) was conducted by treating 1-(dicarbonyl-η5-pentamethylcyclopentadienylferrio)-2-(2,4,6-tri-tert-butylphenyl)diphosphene (7) with an equimolar amount of 2-diisopropylamino-1-phosphaethyne (2b). Monitoring the reaction by 31P{1H} NMR evidenced the formation of trans-1,2-dihydro-1-(dicarbonyl-η5-pentamethylcyclopentadienylferrio)-2-(2,4,6-tri-tert-butylphenyl)-4-diisopropylamino-1,2,3-triphosphete (8) as an intermediate, which in solution slowly isomerized to the final product 9.

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2,4,6‐Tri‐tert‐butyl‐1,3,5‐triphosphabenzene 4 reacts with phosphaalkynes P≡C–R [R = tBu (5a), tPen (5b)] at room temperature in a formal [4 + 2] cycloaddition to yield the corresponding 1,3,5,7‐tetraphosphabarrelene derivatives 8a and 8b, respectively. The analogous reaction of 4 with the aminophosphaethyne P≡C–N(iPr)2 (9) unexpectedly leads to the 1,3,4,7‐tetraphosphasemibullvalene derivative 10 as the only product. The single‐crystal X‐ray analysis of 10 exhibits a diphosphirane unit with a very long PP distance of 2.274(1) Å together with a large extension of the PCP angle in the three‐membered ring to 75.3(1)°.

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