The uranium metallocenes [eta 5-1,3-(Me3E)(2)C5H3](2) UMe2 (E = C, Si) react with NH3 to give the dimers {[eta 5-1,3-Me3E)(2)C5H3](2)U}2(mu-NH)(2) (E = C(1), Si (2)) but with p-toluidine to give the monomeric diamides [eta(5)-1,3-(Me3E)(2)C5H3](2)U(NH-p-tolyl)(2) (E=C (3), Si (4)) do not eliminate p-toluidine but sublime intact at 140 degrees C in a vacuum. The uranium metallocene [eta 5-1,2,4-(Me3C)(3)C5H2](2)UMe2 reacts with the RNH2 to yield [eta(5)-1,2,4-(Me3C)(3)C5H2](2)U(NHR)(2) (R = Me (8), PhCH2 (9), p-tolyl (10)), which are isolated as crystalline solids. In benzene solution these diamides are in equilibrium with [eta(5)-1,2,4-(Me3C)(3)C5H2](2)U = NR2, which may be isolated pure when R is Me (11) or p-tolyl (12), and the primary amine. The monomeric imide (eta(5)-1,2,4-(Me3C)(3)C5H2](2)U = NMe (11) reacts with R'C CR' to yield the cycloaddition products [eta(5)-1,2,4-(Me3C)(3)C5H2](2)U - [N(Me)C(R')] (R' = Me (15), Ph (16), which react with excess MeNH2 to regenerate the diamide [eta 5-1,2,4-(Me3C)(3)C5H2](2)U(NHMe)(2) (8) and MeN = C(R')CH(R'). The methylimide, [eta(5)-1,2,4-(Me3C)(3)C5H2](2)U = NMe (11), does not react with Me3SiX reagents; a model is proposed that rationalises this reactivity pattern.

High-yield preparations of the uranium metallocenes [1,3-(Me3Si)(2)C5H3](2)UCl2 (Cp "2UCl2) and [1,3-(Me3C)(2)C5H3](2)UCl2 (Cp double dagger(2)UCl(2)) have been developed from the reaction of UCl4 and the corresponding magnesocenes Cp "Mg-2 and Cp double dagger(2)Mg in diethyl ether. The chloride ligands can be exchanged with either Me3SiBr or Me3SiI to give the uranium metallocene bromides or iodides. The fluorides were prepared by the reaction of BF3. OEt2 with Cp "U-2(NMe)(2), Cp double dagger(2)U(OMe)(2), or Cp double dagger(2)UMe(2). The crystal, structures of Cp "2UCl2, Cp double dagger(2)UCl(2), Cp "2UMe2, Cp double dagger(2)UF(2), and dimeric (Cp "2UF2)(2) are reported. The idealized symmetry of the monomers Cp "2UX2 and Cp double dagger(2)UCl(2) is C-2v when X is F, Cl, or Br and C-2 when X is I or Me; in the dimer, the idealized symmetry is C-i. This preference is rationalized by intramolecular and intermolecular steric effects. The solution ring conformations and intramolecular exchange processes have been studied by variable-temperature H-1 NMR spectroscopy. In all cases, the low-temperature limiting spectra are consistent with the idealized symmetry observed in the solid state.

Synthesis of remarkably air-, moisture-, and heat-stable tungsten(VI) imido alkylidenes has been achieved by addition of potassium hydrotris(1-pyrazolyl)-borate (KTp) or potassium hydrotris(3,5-dimethyl-1-pyrazolyl)borate (KTp') to W(CHC(CH3)2Ph)(NAr)(OTf)2-(DME) (1; Ar = 2,6-i-Pr2-C6H3, OTf = SO3CF3, DME = CH3OCH2CH2OCH3). Addition of KTp' to 1 yielded [Tp'W-(CHC(CH3)2Ph)(NAr)(pyrH)][OTf] (2; pyrH = 3,5-dimethylpyrazole), which is the first crystallographically characterized d0 cationic alkylidene. Compound 1 reacts with KTp to give TpW(CHC(CH3)2Ph)(NAr)(OTf) (5), which, in the presence of AlCl3, catalyzes the ring-opening metathesis polymerization of cyclooctene, yielding high-molecular-weight poly(1-octenylene) (M(n) = 98 900, M(w)/M(n) = 1. 2).

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