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-

Several products resulting from the condensation of the bis‐(trimethylsilyl)ylide 1 with PCl3 serve as synthetic equivalents of a phosphoniophosphaethyne. Cycloaddition reactions with azides lead to phosphonio‐1,2,3,4‐triazaphosphole cations 5, 7 and to the zwitterionic phosphonio‐1,2,3,4‐triazaphospholide 6. The latter readily undergoes a cycloreversion yielding a phosphoranediyl diazomethane 12 as intermediate. Its cycloaddition affords the diphosphonio‐1,2,4‐diazaphosholide chloride 9a as the final product. 9a is a remarkably stable and unreactive derivative of the two‐coordinate phosphorus. By HCL it is protonated at a nitrogen atom to give dication without any tendency to associate with the chloride counterions.

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P-CI bond dissociationThe 1,3-dichloro-and 1,3-dibromo-substituted title compounds 5b, c result from the condensation of triphenylphosphonium-bis(trimethylsily1)methylide 1 with PC13 and PBr3. The chloro derivative 5b undergoes a variety of single and double substitution reactions, of which some have merely been tested and others have been made prepardtive use of. The substitution of one chloride by an ylidyl, dmino, phosphino, or metal carbonyl group is accompanied by the spontaneous dissociation of the other P-CI bond to yield the 1,3-diphosphetenium chlorides 14/15, 19, 31 and 36, respectively. In other cases (9, 27, 32) dissociat.ion can be achieved by means of a Lewis acid. Protonation of 5b occurs at the carbon ring members; methylation and formation of transition metal complexes occurs at a phosphorus ring member. PCL3 degrades the four-membered ring of 5b or enlarges the ring to yield 3,5-diphosphoranediyl-l,2,4-triphospholane derivatives 39, 40, 42. px2 PXZ px3 SiMe, The alternating combination of the divalent units Ph3PC Ph3P$siMe3 ~_ t px3 Ph3P=( -----+ ph3p=( -Me,Si X 2c, X = B r 3c, X = Br PXZ and PX (X = CI, Br) leads to a new class of oligomers, (Ph3PCPX),, (n = 2, 3, 4)''The characteristic feature of these compounds is the easy dissociation of the PX bonds, with the tendency towards ionic structures increasing with increasing n. The four-membered cyclic dirners, 1 ;3-dihalo-2b, X = C I 3b, X = C I 2,4-bis( triphenylphosphoranediy1)-1 ,3-diphosphetanes are not dissociated. As will be shown here, however, their substitution may be accompanied by dissociation. Halogensubstituted 1,3-DiphosphetanesThe bis(trimethylsily1)ylide 1 reacts with phosphorus trichloride and tribromide in two separate steps lo give the dihalophosphino (trimethylsily1)ylides 2 and the bis(diha1ophosphino)ylides 3[s.61. The chlorosubstituted diphosphelane 5b results from the slow self-condensation of 2b[1.31. To obtain the bromosubstituted diphosphetane 5c, the bis-(trimethylsily1)ylide 1 is condensed with 3~1~1.The latter reaction necessarily passes through an intermediate of type 4, and although the reaction leading from 2b to 5b might instead involve an intramolecular elimination of Me3SiC1 from 2b and thc dimerization of the resulting intermediate, it seems more likely that this reaction also proceeds by an intermolecular condensation and thus also passes through an intermediate of type 4.An X-ray structure analysis of 5b['l shows an almost planar arrangement of the four ring mcmbers and the two exocyclic phosphorus atoms. The two P-C1 bonds are cis to each othe1-1~1 and virtually eclipsed to the filled pz orbitals at the ylidic carbon atoms, with Cl-P-C-P dihedral angles of 82 and 69" for one chlorine atom and 83 and 70" for the other. This conformation favors charge transfer to the5C, X = Br antibonding orbitals of the P-CI bonds (negative hyper-coiijugation~*-"]) and results in extrcmely long PFCI bonds. of 220.4(4) and 225.1(3) pin["] as compared to 204 pm in PCI3. The chemical behaviour of 5 b ha5 been characterized by ...

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Drdicrirrd to Profissov Rolf' Huisgen on ilic~ occusion of'liis 75th hivtlirlq~Triphenylphosphonium ylidyl substituents Ph, exert ;I stronger + M effect than the corresponding isolobal amino groups and, for example, lead to spontaneous dissociation of halophosphanes.[1.21 In the same way, the ylidediyl unit Ph,P=C=, which is isolobal to an imino group, is expected to form halophosphaalkenes 1 (X = CI, Br); upon dissociation, 1 would yield the triphenylphosphoniophosphaethyne ion I LThe attempted synthesis of 1 actually affords its oligomers (Ph,PCPX),,. ( n = 2. 3, 4). They are related to cyclophosphazanes, ['l (RNPX),,. by a formal RN/Ph,PC exchange,[" and consequently, similar symmetrical ring structures are predicted. However. such an analogous structure is only encountered for n = 2. whereas partial dissociation leads to structures of lower symmetry for n = 3, 4.Treatment of bis(trimethylsilyl)ylide 3 with phosphorus trichloride or phosphorus tribromide affords the dihalophosphanyl ylides 4 and 5 (Scheme I).['] The mixed trimethylsilyland dihalophosphanyl-substituted ylides 4 (route 1) are isolable; in solution, however, self-condensation (route 3) takes place to afford the oligomers 6-8. Similarly, the condensation of bis(dihalophosphany1)ylide 5 with bis(trimethylsi1yl)ylide 3 (route 4) yields oligomers 6 and 7.

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