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A pseudo seven-coordinate central phosphorus atom is observed in the phosphanes 1 and 2. In solution this coordination is retained in the rigid phosphane 1, whereas in 2 an equilibrium exists between the opened and closed forms; as a result, MeI reacts with 1 at the P atom, but with 2 at the N atom.
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Chuit et al. (1993) studied this question.
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