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The bis-phosphonio-1,2,4-diazaphospholide salt (1[Cl]) reacts with complex boron hydrides under selective extrusion of one PPh 3 moiety to give borane adducts of a novel zwitterionic phosphonio-1,2,4-diazaphospholide. Both the Et 3 B adduct 2b and the free zwitterionic heterocycle 3, which was liberated by further reaction of 2b with NEt 3 , were characterized by spectroscopic data and 2b, as well, by a single crystal X-ray diffraction study. The comparison of the structural data with those of a neutral 1,2,4-diazaphosphole and a lithium-1,2,4-diazaphospholide which was formed by deprotonation of the parent 1,2,4-diazaphosphole 4a discloses trends in endocyclic bonding distances which can be rationalized in terms of a charge dependent shift in the π-electron distribution. First studies of the co-ordination properties reveal for Ein Zwitterionisches Phosphonio-1,2,4-Diazaphospholid und Neutrales 1,2,4-Diazaphosphol ؊ Eine Vergleichende Studie von Molekülstrukturen und KoordinationseigenschaftenInhaltsübersicht. Das Bis-phosphonio-1,2,4-diazaphospholid-Salz (1[Cl]) reagiert mit komplexen Borhydriden unter selektiver Abspaltung einer PPh 3 -Gruppe zu Boranaddukten eines neuen zwitterionischen Phosphonio-1,2,4-diazaphospholids. Sowohl das Et 3 B-Addukt 2b als auch der freie zwitterionische Heterocyclus 3, der durch weitere Reaktion von 2b mit NEt 3 freigesetzt wurde, wurden durch spektroskopische Daten und 2b zusätzlich durch eine Einkristall-Röntgenstrukturanalyse charakterisiert. Der Vergleich der Strukturdaten mit denen eines neutralen 1,2,4-Diazaphosphols und eines Lithium-1,2,4-diazaphospholids, das durch Deprotonierung der Stammverbindung 1,2,4-Diazaphosphol 4a erhalten wurde, zeigen Trends in den endocyclischen Bindungslängen, die im Sinne ladungsabhängiger Variationen der π-Elektronenvertei-

The neutral monophosphoniobenzo[c]phospholides 2 and 3 were selectively prepared by reduction of bis(triphenylphosphonio)benzophospholide chloride 1[Cl]. Compound 3 was further converted into the borane adduct 6 and the thioxophosphorane 7. All products were characterised by spectroscopic methods and X-ray diffraction. The most notable features of the molecular structures are the significantly different distances of the two P−C bonds adjacent to the two-coordinate phosphorus atom (P2−C1 175.0−176.6, P2−C3 171.7−173.5 pm), and a short exocyclic C−P(phosphonio) bond (C1−P1 172.5−173.7 pm) relative to cationic derivatives such as 1. Structure comparisons revealed further variations in the bond lengths, which can be related to the changes in

1-Triphenylphosphoniobenzo[c]phospholide 1 reacts with [M(CO)(5)Br] (M = Mn, Re) and [Mn(CO)(3)(naphthalene)][BF(4)] to give complexes cis-[M(CO)(4)(1)Br] (5 a,b) and [Mn(CO)(3)(1)][BF(4)] (6 a[BF(4)]), respectively, featuring eta(1)(P)- and eta(5)(pi)-coordination of the phosphole ring. The corresponding reactions with [M(2)(CO)(10)] proceed with conservation of the metal-metal bond and yield, depending on the reaction temperature, dinuclear complexes [M(2)(CO)(8)(1)] (M=Mn, 7 a) or [M(2)(CO)(6)(1)(2)] (M=Mn, Re, 8 a,b) with mu(2)-bridging eta(1)(P):eta(2)(Pdbond;C) coordination of the phosphole moiety. All complexes formed were characterized by spectroscopic data; 5 b, 6 a[BF(4)], and 8 a,b were characterized by X-ray diffraction studies as well. The structural and (31)P NMR data of the dinuclear manganese complex 8 a suggest that the interaction between the metal atoms and the eta(2)-bound Pdbond;C double bond moieties is dominated by the L-->M charge-transfer contribution; this hints at a very low back-donation ability of the central M(2)(CO)(6) fragment. Investigation of the reactions of the Mn complexes 6 a and 8 a with Mg or ferrocenium hexafluorophosphate ([Fc][PF(6)]), respectively, revealed that the chemically reversible mutual interconversion between both species was feasible. Likewise, oxidation of the rhenium complex 8 b with [Fc][PF(6)] gave spectroscopic evidence for the formation of a Re analogue of 6 a. Electrochemical studies suggested that the oxidation 8 a-->2 6 a involves two consecutive single-electron-transfer steps, the first of which is electrochemically reversible and produces a metastable radical cation that is detectable by ESR spectroscopy. The mutual interconversion between 6 a and 8 a represents the first case of a reversible coordination isomerization of a phosphaarene that is triggered by a redox process and might stimulate further studies directed at the use of dinuclear phosphaarene complexes in redox-catalysis.