fluorine atoms from a variety of perfluoroolefins including perfluoro-2,4-dimethyl-3-ethylpent-2-ene, perfluoro-2,3-dimethylpent-2-ene, and perfluorocyclohexene. A similar abstraction of fluorine atoms from perfluorobenzene is much slower. However, visible light enhances the rate of fluorine abstraction from perfluorobenzene by MCp*2-OEt2 (M = Yb, Eu, Sm). Qualitative observation shows that the relative fluorine abstraction reactivity of the four lanthanoid complexes increases with increasingly negative reduction potential for reasonably unhindered fluoroolefin substrates. The Yb(III)/Yb(II) reduction potential of YbCp'2 solvated in acetonitrile is determined here to be -1.65 V (relative to ferrocene) by cyclic voltammetry. The fully characterized organometallic products from the fluorine atom abstraction reactions are solvated trivalent lanthanoid fluorides MCp*2F-L (M = Yb, Eu, Sm; L = diethyl ether or THF) and YbCp'2F-THF. The molecular structures of YbCp*2F-OEt2 and YbCp*2F-THF determined by X-ray crystallography reveal the first terminal lanthanoid-fluoride bonds. However, three different environments for bridging lanthanoid-fluoride bonds are seen in the X-ray determined structure of the trivalent pentaytterbium cluster [Yb5Cp*6(/u4-F)(M3-F)2(M-F)6], which is a secondary product from the further reaction of YbCp*2F with perfluoroolefins. From the X-ray data of these three trivalent ytterbium compounds the averaged Yb-F distances are 2.02 A for the terminal Yb-F bonding mode, 2.20 A for an F atom bridging between two Yb atoms, and 2.37 A for an F atom bridging between three of four Yb atoms.

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Low-valent rhenium-oxo compounds of the form Re(0)I(RC=CR')2 have been prepared by the reaction of ReO(I)3(AsPh3)2 with acetylenes (RCCR' = MeCCMe (1), EtCCEt (4), MeCCH (5), Me3CCCH (6), PhCCMe (7), MeCC(CH2)4CCMe ( 8)). Compound 1 crystallizes in space group P2Jc, a = 9.749 (3) k,b = 7.307 (3) Á, c = 16.106 (7) Á, ß = 106.42 (3)°, Z = 4. The structure was refined to final residuals R = 0.027, Rw = 0.029. The rhenium adopts a slightly distorted tetrahedral geometry in which the acetylene ligands occupy two vertices. There is a short, multiple rhenium-oxygen bond of 1.697 (3) Á. The chloride and bromide analogues of 1 can be prepared by addition of 2-butyne to solutions of ReO(X)3(EPh3)2 (X = Cl, Br; E = P, As). Reactions of 1, 6, or 7 with Ag+ in the presence of another ligand form ReO(02CMe)(MeCCMe)2 or [ReO(RCCR')2L]SbF6 (L = py, bpy, PPh3). A survey of metal complexes with terminal oxo ligands reveals the virtual absence of compounds containing electrons in metal-oxygen antibonding orbitals. In an octahedral coordination geometry, this normally restricts stable oxo complexes to d°, d1, or d2 electron counts. Extended Htickel calculations indicate that the rhenium(III), d4, oxo compounds reported here are stable because they adopt a pseudotetrahedral structure, in which there is essentially a threefold axis about the Re-O bond. In this geometry, four d electrons can be accommodated without populating rhenium-oxygen antibonding levels.

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