The reaction of Cp'2CeH (Cp' = 1,2,4-(Me3C)3C5H2 ) with MeOSiMe3 gives Cp'2CeOMe and HSiMe3 and the reaction of the metallacycle, Cp'[(Me3C)2C5H2C(Me) 2CH2]Ce, with MeOSiMe3 yields Cp'2CeOCH2SiMe3, formed from hypothetical Cp'2CeCH2OSiMe3 by a [1, 2] shift also known as a silyl-Wittig rearrangement. Although both cerium products are alkoxides, they are formed by different pathways. DFT calculations on the reaction of the model metallocene, Cp2CeH, and MeOSiMe3 show that the lowest energy pathway is a H for OMe exchange at Ce that occurs by way of a sigma-bond metathesis transition state as SiMe3 exchanges partners. The formation of Cp2CeOCH2SiMe3 occurs by way of a low activation barrier [1, 2] shift of the SiMe3 group in Cp2CeCH2OSiMe3. Calculations on a model metallacycle, Cp[C5H4C(Me)2CH2]Ce, show that the metallacycle favors CH bond activation over sigma-bond metathesis involving the transfer of the SiMe3 group in good agreement with experiment. The sigma-bond metathesis involving the transfer of SiMe3 and the [1, 2] shift of SiMe3 reactions have in common a pentacoordinate silicon at the transition states. A molecular orbital analysis illustrates the connection between these two Si-O bond cleavage reactions and traces the reason why they occur for a silyl but not for an alkyl group to the difference in energy required to form a pentacoordinate silicon or carbon atom in the transition state. This difference clearly distinguishes a silyl from an alkyl group as shown in the study of "Pyrolysis of Tetramethylsilane Yielding Free d-orbitals by Seyferth and Pudvin in ChemTech 1981, 11, 230-233".

Addition of diethylether to [1,2,4(Me3C)3C5H2]2CeH, abbreviated Cp'2CeH, gives Cp'2CeOEt and ethane. Similarly, di-n-propyl- or di-n-butylether gives Cp'2Ce(O-n-Pr) and propane or Cp'2Ce(O-n-Bu) and butane, respectively. Using Cp'2CeD, the propane and butane contain deuterium predominantly in their methyl groups. Mechanisms, formulated on the basis of DFT computational studies, show that the reactions begin by an alpha or beta-CH activation with comparable activation barriers but only the beta-CH activation intermediate evolves into the alkoxide product and an olefin. The olefin then inserts into the Ce-H bond forming the alkyl derivative, Cp'2CeR, that eliminates alkane. The alpha-CH activation intermediate is in equilibrium with the starting reagents, Cp'2CeH and the ether, which accounts for the deuterium label in the methyl groups of the alkane. The one-step sigma-bond metathesis mechanism has a much higher activation barrier than either of the two-step mechanisms.

The reaction between [1,2,4-(Me3C)3C5H2]2CeH, referred to as Cp'2CeH, and CH3X where X is Cl, Br, I, OMe and NMe2, are described. The reactions fall into three distinct classes. Class a, where X = Cl, Br and I rapidly form Cp'2CeX and CH4 without formation of identifiable intermediates in the 1H NMR spectra. Class b, where X = OMe proceeds rapidly to Cp'2Ce(eta2-CH2OMe) and H2 and then to Cp'2CeOMe and CH4. The methoxymethyl derivative is sufficiently stable to be isolated and characterized and it is rapidly converted to Cp'2CeOMe in presence of BPh3. Class c, where X = NMe2 does not result in formation of Cp'2CeNMe2, but deuterium labeling experiments show that H for D exchange occurs in NMe3. Density functional calculations DFT(B3PW91) on the reaction of (C5H5)2CeH, referred to as Cp2CeH, and CH3X show that the barrier for alpha-CH activation, resulting in formation of Cp2Ce(eta2-CH2X), proceeds with a relatively low activation barrier (DeltaG++) but the subsequent ejection of CH2 and trapping by H2 has a higher barrier; the height of the second barrier lies in the order F, Cl, Br, I < OMe << NMe2, consistent with the experimental studies. The DFT calculations also show that the two-step reaction, which proceeds through a carbenoid intermediate, has a lower barrier than a direct one-step sigma bond metathesis mechanism. The reaction of Cp2CeCH2OMe and BPh3 is calculated to be a low barrier process and the ylide, CH2(+)BPh3(-), is a transition state and not an intermediate.

The experimental reaction between [1,2,4-(Me3C)3C5H2]2CeCH2Ph and CH3X, X = F, Cl, Br, and I, yields the metathetical exchange products, [1,2,4-(Me3C)3C5H2]2CeX and CH3CH2Ph. The reaction is complicated by the equilibrium between the benzyl derivative and the metallacycle [[1,2,4-(Me3C)3C5H2] [(Me3C)2C5H2C(CH3)2CH2]Ce, plus toluene since the metallacycle reacts with CH3X. Labelling studies show that the methyl group of the methylhalide is transferred intact to the benzyl group. The mechanism, as revealed by DFT calculations on (C5H5)2CeCH2Ph and CH3F, does not proceed by way of a four-center mechanism, (sigma-bond metathesis) but a lower barrier process involves a haptotropic shift of the Cp2Ce fragment so that at the transition state the para-carbon of the benzene ring is attached to the Cp2Ce fragment while the CH2 fragment of the benzyl group attacks CH3F that is activated by coordination to the metal ion. As a result the mechanism is classified as an associative interchange process.

The reaction between monomeric bis(1,2,4-tri-t-butylcyclopentadienyl)cerium hydride, Cp'2CeH, and several hydrofluorobenzene derivatives is described. The aryl derivatives that are the primary products, Cp'2Ce(C6H5-xFx) where x = 1,2,3,4, are thermally stable enough to be isolated in only two cases, since all of them decompose at different rates to Cp'2CeF and a fluorobenzyne; the latter is trapped by either solvent when C6D6 is used or by a Cp'H ring when C6D12 is the solvent. The trapped products are identified by GCMS analysis after hydrolysis. The aryl derivatives are generated cleanly by reaction of the metallacycle, Cp'((Me3C)2C5H2C(Me2)CH2)Ce, with a hydrofluorobenzene and the resulting arylcerium products, in each case, are identified by their 1H and 19F NMR spectra at 20oC. The stereochemical principle that evolves from these studies is that the thermodynamic isomer is the one in which the CeC bond is flanked by two ortho-CF bonds. This orientation is suggested to arise from the negative charge that is localized on the ipso-carbon atom due to Co(delta+)-Fo(delta-) polarization. The preferred regioisomer is determined by thermodynamic rather than kinetic effects; this is illustrated by the quantitative, irreversible solid-state conversion at 25oC over two months of Cp'2Ce(2,3,4,5-C6HF4) to Cp'2Ce(2,3,4,6-C6HF4), an isomerization that involves a CeC(ipso) for C(ortho)F site exchange.

The sterically demanding 1,2,4-tri-t-butylcyclopentadienyl ligand (1,2,4-(Me3C)3C5H2, hereafter Cp') has been used to prepare monomeric cerium metallocenes, Cp 2CeX (X = Cl, I, OSO2CF3), which are used to synthesize the benzyl, Cp'2CeCH2C6H5. The benzyl is a useful starting material for preparing other complexes in the Cp'2CeZ system (Z = BF4, F, NH2, C6H5, H). X-ray crystal structures of Cp'2CeOSO2CF3, Cp'2CeF, Cp'2CeCH2C6H5, and Cp'2CeH are presented. The benzyl slowly decomposes in solution to toluene and a metallacycle, [Cp'][(Me3C)2C5H2(CMe2CH2)]Ce. The ring CMe3 groups of both the metallacycle and the hydride, Cp'2CeH, can be fully deuterated by prolonged exposure to C6D6, providing a useful labeling tool in mechanistic studies.The hydride activates C-F and/or C-H bonds in fluorobenzenes, C6HxF6-x , x = 0-5. The reactions are selective, with the selectivity depending on the presence of two fluorines ortho to the reaction site more than on the type of bond activated. Complexes of the type Cp'2CeC6HxF5-x , x = 0-4, are formed as intermediates, which slowly decompose in solution to Cp'2CeF and fluorobenzynes, C6HxF4-x, x = 0-4, which are trapped. The rate at which Cp'2CeC6HxF5-x complexes decompose increases as the number of fluorines decreases. Complexes with one ortho-fluorine decompose much faster than those with two ortho-fluorines. The metallacycle activates only C-H bonds in fluorobenzenes, permitting the synthesis of specific Cp'2CeC6HxF5-x complexes. The crystal structure of Cp'2CeC6F5 is presented. The hydride and the metallacycle react with fluoromethanes, CH4-xFx, x = 1-3, through postulated Cp'2CeCH3-xFx intermediates to generate Cp'2CeF and other products. The other products, CH4, tri-t-butylbenzenes, tri-t-butylfluorobenzenes, and a presumed metallocene cerium fluoride with one Cp' and one (Me2EtC)(Me3C)2C5H2 ligand, suggest a decomposition pathway for Cp'2CeCH3-xFx , x = 1-3, that involves carbenes or carbenoids, which are trapped. The hydride polymerizes ethylene, but hydrogenates other olefins. The metallacycle activates C-H bonds in olefins and aromatics to generate new complexes with Ce-C bonds. The hydride reacts with one equivalent of CO in pentane to generate (Cp'2Ce)2CH2O, whose crystal structure shows the presence of a bridging dianionic formaldehyde ligand. (Cp'2Ce)2CH2O reacts H2 to give the hydride and Cp'2CeOMe, or with a mixture of H2 and CO to generate Cp'2CeOMe exclusively. (Cp'2Ce)2CH2O or the hydride can react with an additional equivalent of CO to generate dimeric enediolate, (Cp'2CeCHO)2.

Addition of CO to [1,2,4-(Me3C)3C5H2]2CeH, Cp'2CeH, in toluene yields the cis (Cp'2Ce)2(mu-OCHCHO), in which the cis enediolate group bridges the two metallocene fragments. The cis enediolate quantitatively isomerizes intramolecularly to the trans-enediolate in C6D6 at 100oC over seven months. When the solvent is pentane, Cp'2Ce(OCH2)CeCp'2 forms, in which the oxomethylene group or the formaldehyde dianion bridges the two metallocene fragments. The cis enediolate is suggested to form by insertion of CO into the Ce-C bond of Cp'2Ce(OCH2)CeCp'2 generating Cp'2CeOCH2COCeCp'2. The stereochemistry of the cis-enediolate is determined by a 1,2-hydrogen shift in the OCH2CO fragment that has the OC(H2) bond anti periplanar relative to the carbene lone pair. The bridging oxomethylene complex reacts with H2, but not with CH4, to give Cp'2CeOMe, which is also the product of the reaction between Cp'2CeH and a mixture of CO and H2. The oxomethylene complex reacts with CO to give the cis enediolate complex. DFT calculations on C5H5 model metallocenes show that the reaction of Cp2CeH with CO and H2 to give Cp2CeOMe is exoergic by 50 kcal mol-1. The net reaction proceeds by a series of elementary reactions that occur after the formyl complex, Cp2Ce(eta-2 CHO), is formed by further reaction with H2. The key point that emerges from the calculated potential energy surface is the bifunctional nature of the metal formyl in which the carbon atom behaves as a donor and acceptor. Replacing H2 by CH4 increases the activation energy barrier by 17 kcal mol-1.

Reduction of the uranium metallocene, [eta5-1,2,4-(Me3C)3C5H2]2UCl2 (1), Cp'2UCl2, in the presence of 2,2'-bipyridyl and sodium naphthalene gives the dark green metallocene complex, Cp'2U(bipy) (6), which reacts with p-tolylazide or pyridine-N-oxide to give Cp'2U=N(p-tolyl) (7) or Cp'2U(O)(py) (8), respectively. The Lewis acid, BPh3, precipitates Ph3B(py) and gives the base-free oxo, Cp'2UO (10), which crystallizes from pentane. The oxometallocene 10 behaves as a nucleophile with Me3SiX reagents but it does not exhibit cycloaddition behavior with acetylenes, suggesting that the polar resonance structure, Cp'2U+-O- dominates the double bond resonance structure Cp'2U=O.