The ylidyl substituent of the chlorophosphane Ph3P(DOUBLE BOND)CAr(SINGLE BOND)PRCl exerts a strong influence on the P(SINGLE BOND)Cl bond. An X‐ray structure investigation of the representative with Ar(DOUBLE BOND)Ph, R(DOUBLE BOND)Me reveals the longest P(SINGLE BOND)Cl bond ever observed for an acyclic chlorophosphine (226.2(1) pm). It is connected to a conformation that allows an effective negative hyperconjugation. The ylidyl chlorophosphanes with an amino group R are covalent in benzene but become more or less ionic in dichloromethane solution. The solvent‐dependent dissociation equilibrium can be followed by 31P NMR spectra. In case of an enamine‐derived ylidyl chlorophosphane, the equilibrium shifts almost completely from the covalent to the ionic side within a rather narrow range of solvent composition (20 to 70 vol % dichloromethane). © 1996 John Wiley & Sons, Inc.

From the reaction of trimethylsilyl ylides with AsCl3 the dichloroarsanyl ylides 2b and 5b are obtained. As shown by X-ray structure determination, their AsCl2 groups deviate systematically from the symmetric orientation. This conformation enables an effective charge transfer from the ylide moiety to one of the As-Cl bonds, which as a consequence is up to 15 pm longer than the other. At the same time the length of the As-C bonds in 2b and 5b indicates a partial double bond. The effects observed here are of the same type as those observed for the corresponding dichlorophosphanyl ylides; they are, however, more pronounced. The 1:1 condensation of the bis(trimethylsilyl) ylide 3 and AsCl3 yields the oligomers (Ph3PCAsCl)2,3,4. The dimer 7b has a diarsetane structure. HCl adds readily to one of its As-C bonds without opening it. The trimer and the tetramer are ionic. The cation of the trimer forms a six-membered ring with a delocalized arsenium/phosphonium charge, the cation of the tetramer forms a barrelane cage with a phosphonio substituent, and the anion is AsCl4- in both cases (10, 13). An arsa-phosphocyanine cation as in 10 is also part of the diphosphonio isoarsindolide tetrachloroarsenate(III) 12. The structures of 7b.HCl, 10, and 13 reflect again an ylide to As-Cl charge transfer. The As-Cl bonds of 13 are by far the longest ones known for a chloroarsine (average 249 pm).

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