CP or CP? One‐electron oxidation of Li/Cl phosphinidenoid complexes led to the discovery of transient P‐chlorophosphanyl complexes 1. Subsequent cross‐coupling and rearrangement or elimination reactions yielded 2 and 3; the latter is the first structurally characterized phosphaquinomethane complex. ESR spectroscopy and DFT calculations support the existence of short‐lived P‐centered radicals.

The synthesis of P-F phosphane metal complexes [(CO)5M{RP(H)F}] 2a-c (R = CH(SiMe3)2; a: M = W; b: M = Mo; c: M = Cr) is described using AgBF4 for a Cl/F exchange in P-Cl precursor complexes [(CO)5M{RP(H)Cl}] 3a-c; thermal reaction of 2H-azaphosphirene metal complexes [(CO)5M{RP(C(Ph)═N}] 1a-c with [Et3NH]X led to complexes 3a-c, 4, and 5 (M = W; a-c: X = Cl; 4: X = Br; 5: X = I). Complexes 2a-c, 3a-c, 4, and 5 were deprotonated using lithium diisopropylamide in the presence of 12-crown-4 thus yielding Li/X phosphinidenoid metal complexes [Li(12-crown-4)(Et2O)n][(CO)5M(RPX)] 6a-c, 7a-c, 8, and 9 (6a-c: M = W, Mo, Cr; X = F; 7a-c: M = W, Mo, Cr; X = Cl; 8: M = W; X = Br; 9: M = W; X = I). This first comprehensive study on the synthesis of the title compounds reveals metal and halogen dependencies of NMR parameters as well as thermal stabilities of 6a, 7a, 8, and 9 in solution (F > Cl > Br > I). DOSY NMR experiments on the Li/F phosphinidenoid metal complexes (6a-c; M = W, Mo, Cr) rule out that the cation and anion fragments are part of a persistent molecular complex or tight ion pair (in solution). The X-ray structure of 6a reveals a salt-like structure of [Li(12-crown-4)Et2O][(CO)5W{P(CH(SiMe3)2)F}] with long P-F and P-W bond distances compared to 2a. Density functional theory (DFT) calculations provide additional insight into structures and energetics of cation-free halophosphanido chromium and tungsten complexes and four contact ion pairs of Li/X phosphinidenoid model complexes [Li(12-crown-4)][(CO)5M{P(R)X}] (A-D) that represent principal coordination modes. The significant increase of the compliance constant of the P-F bond in the anionic complex [(CO)5W{P(Me)F}] (10a) revealed that a formal lone pair at phosphorus weakens the P-F bond. This effect is further enhanced by coordination of lithium and/or the Li(12-crown-4) countercation (to 10a) as in type A-D complexes. DFT calculated phosphorus NMR chemical shifts allow for a consistent interpretation of NMR properties and provide a preliminary explanation for the "abnormal" NMR shift of P-Cl derivatives 7a-c. Furthermore, calculated compliance constants reveal the degree of P-F bond weakening in Li/F phosphinidenoid complexes, and it was found that a more negative phosphorus-fluorine coupling constant is associated with a larger relaxed force constant.

Reactivity studies of Li/Cl phosphinidenoid W(CO) 5 complexes 2a,b toward various iodine compounds are reported. Transiently generated complexes 2a,b yielded no selective reactions with 3-, 9-, and 9,12-diiodo o-carbaboranes 3a-c, whereas clean transfer-iodination reaction occurred with C-iodo-substituted o-carbaboranes 3d,e, thus giving chloro(iodo)phosphane complex 6a in the case of 2a. Complex 2a was also reacted with iodo(phenyl)acetylene to yield complexes 6a, 8, and 9 in competing reactions. An independent pathway to chloro(iodo)phosphane complexes 6a,b was reaction of complexes 2a,b with elemental iodine at low temperature. All compounds were unambiguously characterized by elemental analysis, multinuclear NMR, IR, MS studies, and, in the case of 6a and 9, single-crystal X-ray diffraction.

Oxidation of Li/X phosphinidenoid complex 2, obtained via selective deprotonation from the P-H precursor 1, with [Ph(3)C]BF(4) led to the formation of two P-F substituted diorganophosphane complexes 6,7; the latter tautomer 7 formed via H-shift from 6. In contrast, oxidation of 2 with [(p-Tol)(3)C]BF(4) led to three major and one minor intermediates at low temperature, which we tentatively assign to two pairs of P-C atropisomers 10 a,a' and 10 c,c' and which differ by the relative orientations of their CH(SiMe(3))(2) and W(CO)(5) groups. Conversion of all isomers led finally to complex 11 having a ligand with a long P-C bond to the central trityl* carbon atom, firmly established by single-crystal X-ray analysis. DFT calculations at the B3LYP/def2-TZVPP//BP86/def2-TZVP level of theory on real molecular entities revealed the structures of the in situ formed combined singlet diradicals (4+5 and 5+9) and the nature of intermediates on the way to the final product, complex 11. Remarkable is that all isomers of 11 possess relative energies in the narrow energy regime of about 20 kcal  mol(-1). A preliminary study revealed that complex 11 undergoes selective P-C bond cleavage at 75 °C in toluene solution.

Professor Michael F. Lappert gewidmetHauptgruppenelementverbindungen mit einem oder mehreren ungepaarten Elektron(en) haben sich in den letzten Jahren als ein faszinierendes Forschungsgebiet entwickelt. [1,2] Der Durchbruch bei Radikalen mit einem niederkoordinierten Phosphorzentrum [3] gelang Lappert et al. mit der Synthese des ersten stabilen Derivats vom Typ I (R = CH(SiMe 3 ) 2 ), [4] das bei der Kristallisation (reversibel) dimerisiert.[5] Diese Verbindung hat als Ligand in Cobalt-und Eisencarbonylkomplexen Verwendung gefunden. [6] In den vergangenen Jahren konnten weitere heteroatomsubstituierte Derivate von II [7] und III [8] synthetisiert werden (Schema 1). Im Fall von III wurden interessante Umlagerungs-und Zersetzungsreaktionen beobachtet. Derivate IV, [9,10] die potenzielle Abgangsgruppen am P-Atom aufweisen, wurden weniger gut untersucht als I-III, und nach unserer Kenntnis ist über ihre Koordinationschemie bislang nichts bekannt. Letztere könnte jedoch von besonderem Interesse sein, da offenschalige Komplexe z. B. als Kontrastmittel in Bildgebungsverfahren in Betracht kommen. [11] Die Erforschung der Li/Cl-Phosphinidenoid-Komplexchemie [12][13][14][15][16] führte nun zur Entdeckung kurzlebiger P- [12,14] in die P-Chlorphosphinidenoid-Komplexe 2 a,b überführt und dann bei tiefer Temperatur mit Trityliumtetrafluoroborat umgesetzt. Langsames Aufwärmen ergab die Komplexe 5 und 6, die durch Säulen-chromatographie isoliert wurden. In Schema 2 ist der vorgeschlagene Reaktionspfad dargestellt, welcher die oxidative Bildung eines Radikalpaars, bestehend aus dem Tritylradikal und den P-Chlorphosphanyl-Komplexen 3 a,b, umfasst. Nach einer C-P-Kupplung, die zu den Komplexen 4 a,b führt, kommt es entweder zu einer H-Verschiebung unter Bildung des Komplexes 5 oder zur HCl-Eliminierung, die Komplex 6 ergibt. Das Auftreten offenschaliger Intermediate 3 a,b wurde Schema 1. Niederkoordinierte Phosphorradikale ohne (I) und mit funktionellen Gruppen wie -NR 2 (II), -PR 2 (III) und -X (IV) am P-Atom. X = OR oder Halogen. Schema 2. Vorgeschlagener Reaktionspfad der Umsetzung der Phosphinidenoidkomplexe 2 a,b mit Trityliumtetrafluoroborat unter Bildung der transienten P-Chlorphosphanyl-Komplexe 3 a,b und letztlich der Komplexe 5 und 6.