Not 1,2,3-triphosphabutadiene but the “butterfly molecule” 3 is formed in the line reaction of 1 and 2. The bond between the bridgehead P atoms of 3 has a length of only 2.136 Å. Presumably 3 is formed by intramolecular [2+2]-cycloaddition of the actually expected triphosphabutadiene (Aryl = 2,4,6-tri-tert-butylphenyl).
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Appel et al. (1988) studied this question.
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