The reaction of phosphonium ylides with phosphorus trihali-to the phosphonio group. In one case the P(II1)-C rotation des has been studied for the synthesis of ylidyl-dihalophos-barrier has been estimated from VT-31P-NMR spectra. By Xphanes (= dihalophosphanyl ylides) Ph3P=CR-PX2 3, X = ray crystallography the structures of 3, R = Me, 2,6-C12C6H,, C1, and 9, X = Br. Compounds 3, R = aryl, are readily prepa-4-NOzC6H5, PC12, of 9, R = Me (two molecules), %Me3, of an red from the phosphonium bromides [Ph3P-CH2R]Br, com-ylidyl-selenophosphonyl dichloride (11 b), and of 12 have pounds 3, R = alkyl, SiMe3 or PC12, and 9 are obtained from been analyzed. They provide representatives for the full silylylides Ph3P=CR-SiMe,, compound 3, R = PPh; results range of rotation from the symmetric conformer with two from the addition of PC1, to the hexaphenylcarbodiphospho-equal P-X bonds to the conformer with one P-X bond perrane. A (P-morpholinoviny1)dichlorophosphane 12 has also pendicular to the PCP plane and with this bond being exbeen prepared. Ylides 3 are oxidized by sulfur and selenium tremely elongated. Thus, they map out the pathway to P-X and are converted to ylidyl-chlorophosphenium (= chloro-bond breaking. On this way the initial charge transfer from phosphaalkenyl-phosphonium) salts [Ph3P-CR=PC1]A1C14 the ylidic carbon to the antibonding P-X orbital ends up in 10. In the ,'P-NMR spectra of 3 and 9 the geminal coupling a TC donation and P-X dissociation. 'Jpp indicates the phosphorus lone pair to be synperiplanar ,,Although direct observation of a molecule along the reaction pathways does not seem feasible, its visualization at least does". Das zu leisten verspricht die Strukturkorrelalions-Hypothese: ,,If a correlation can be found between two independent parameters describing the structure of a given structural fragment in a variety of environments, then the correlation function maps a minimum energy path in the corresponding parameter space" [']. Bei dem hier zu beschreibenden Vorgang handelt es sich um die Heterolyse einer Phosphor(II1)-Halogen-Bindung oder vielmehr um die Einleitung dieser Heterolyse. Zur Wegmarkierung dienen die Strukturen von sieben Dihalogenphosphanen. Die ,,Schnappschusse entlang des Wegs", die sie bieten, sind einerseits hinreichend verschieden und passen andererseits gut genug zusammen, um daraus einen uberzeugenden vorgangsbeschreibenden Film zu machen. Die Reihenfolge, in der die Schnappschiisse dazu angeordnet werden mussen, ist uberdies nicht zufallig, sondern ist in den meisten Fallen einsichtig.Unabhangig von der Art der sonstigen Substituenten sind Phosphor(II1)-halogenide in aller Regel kovalent, und ionische Vertreter sind die seltene Ausnahme. Letztere finden sich dort, WO die bei der Dissoziation am Phosphoratom induzierte Kationladung effektiv auf die ubrige Molekel verteilt werden kann, wie z.B. in bestimmten Azaphos-