Metallocene Ligands Research Articles

Synthesis and bonding in platinum(II) complexes with cyclometalated carbene and isoelectronic bis(phosphino)metallocene ligands

Four cycloplatinated complexes of the type [Pt(C^C*){M(η5-C5H4PPh2)2-κ2P,P'}][PF6]n (M = Fe, n = 1; M = Co, n = 2), where C^C* is 2-(3-methylimidazol-2- ylidene-1-yl-κC2)phenyl-κC1 and similar cyclometalated ligands bearing an additional aromatic ring and phenyl substituents at the imidazol-2-ylidene unit, were prepared and structurally characterized by spectroscopic methods (NMR and ESI MS), elemental analysis, cyclic voltammetry and DFT calculations. Experimental structural parameters and DFT calculations consistently indicate differentiation of the Pt-bound phosphine groups due to unlike trans influence of the phenyl and carbene donor moieties at the cyclometalated ligand in these compounds

Construction of intermolecular σ-hole interactions in rare earth metallocene complexes using a 2,3,4,5-tetraiodopyrrolyl anion.

The generation of noncovalent intermolecular interactions represents a powerful method to control molecular vibrations and rotations. Combining these with the axial ligand field enforced by the metallocene ligand scaffold provides a dual-pronged approach in controlling the magnetic-relaxation pathways for dysprosium-based single-molecule magnets (SMMs). Here, we present the first implementation of 2,3,4,5-tetraiodopyrrole (TIPH) in its anionic form [TIP]- as a ligand in three isostructural rare-earth metal complexes Cp*2RE(TIP) (1-RE, RE = Y, Gd, and Dy; Cp* = pentamethylcylopentadienyl), where the TIP ligand binds through the nitrogen and one iodine atom κ2(N,I) to the metal centre. The shallow potential energy surface of the intermolecular σ-hole interaction yields distortions of the interatomic distances at elevated temperatures which were investigated by variable-temperature SCXRD. 1-RE constitute the first crystallographically characterized molecules containing TIP as a ligand for any metal ion, and 1-Dy is the first SMM that employs the TIP ligand. The structural dependence on temperature allowed the mechanism of magnetic relaxation to be explored through ab initio calculations at different temperatures. The electronic influence of the coordinated iodine substituent was probed via magnetometry and cw-EPR spectroscopy on 1-Gd. To further scrutinize the impact of the iodine substituents on the physical properties, a second set of new complexes Cp*2RE(DMP) (2-RE, RE = Y, and Dy) where DMP = 2,5-dimethylpyrrolyl were synthesized. Here, the DMP ligand binds similarly to the TIP ligand and represents an all-hydrocarbon analogue to 1-RE. 2-Dy constitutes the first SMM bearing a DMP ligand.

Incorporated ferrocene-derivatives endow Ni-based MOF with high-performance for electrochemical detection

Metal-organic frameworks (MOFs) exhibit structural diversity and functional tunability, rendering them a versatile platform for a wide range of applications. Specifically, defects or function groups induced by exotic ligands is an efficient strategy to adjust the catalytic performance. In this study, we explore the impact of incorporating ferrocene derivatives into two Ni-based MOFs and examine their suitability as potential glucose sensors. One strategy involves introducing bidentate 1,1′-ferrocenedicarboxylic acid (1,1′-Fc) into the 3D Ni-1,4-benzenedicarboxylic acid MOF (Fc-Ni-BDC) using a simple single-step hydrothermal method. Another approach involves encapsulating ferrocenemethylamine (Fc-NH2) inside the cavity of 2D conductive Ni-HHTP MOF (HHTP=2,3,6,7,10,11-hexahydroxytriphenylene) driven by electrostatic attraction to form Fc-Ni-HHTP. On account of the synergy between ferrocene derivatives and MOFs, the hybrid MOFs demonstrated remarkable electrochemical performance, characterized by pronounced electrocatalytic reactivity towards glucose oxidation. It is showed that the doped ferrocene derivatives act as electron donors to the MOF active sites, modifying the electronic structure and facilitating the kinetics of the reactions, then an augmentation in the catalytic efficiency has been achieved. This work demonstrates a simple strategy to improve MOF-based electrochemical sensors by incorporating metallocene ligands.

The effect of replacing linker with carboxylic acid ligands on the oxygen evolution performance of metal-organic frameworks

Metal-organic frameworks (MOFs) are versatile platforms for developing various applications, owing to their structural diversity and functional tunability. The catalytic performance of metal-organic frameworks (MOFs) can be improved by introducing defects using monocarboxylate ligands, but the role of these ligands remains unclear. Herein, we show how doping MOFs with electron-withdrawing or electron-donating monocarboxylate ligands affects their oxygen evolution reaction (OER) performance. We further demonstrate that ferrocene derivatives as doped ligands can significantly enhance the OER performance of MOFs, owing to the strong synergistic effects between ferrocene derivatives and MOFs. Different from other doping MOF, ferrocene derivatives doped MOFs (1,1'-Fc-Ni-BDC) exhibit remarkable electrocatalytic activity with a low overpotential of 387 mV at 10 mA cm−2, and possess good electrochemical stability. Density functional theory (DFT) calculations reveal that the active sites are not on ferrocene derivatives, but rather on MOFs, and that ferrocene derivatives facilitate the formation of OER intermediates. Our findings provide new insights into the role of ligands in modulating the OER performance of MOFs. This work offers a new perspective to develop efficient MOF-based electrocatalysts by introducing electronegative metallocene ligands.

Synthesis and Crystallographic Characterization of a Reduced Bimetallic Yttrium ansa-Metallocene Hydride Complex, [K(crypt)][(μ-CpAn)Y(μ-H)]2 (CpAn = Me2Si[C5H3(SiMe3)-3]2), with a 3.4 Å Yttrium-Yttrium Distance.

The reduction of a bimetallic yttrium ansa-metallocene hydride was examined to explore the possible formation of Y-Y bonds with 4d1 Y(II) ions. The precursor [CpAnY(μ-H)(THF)]2 (CpAn = Me2Si[C5H3(SiMe3)-3]2) was synthesized by hydrogenolysis of the allyl complex CpAnY(η3-C3H5)(THF), which was prepared from (C3H5)MgCl and [CpAnY(μ-Cl)]2. Treatment of [CpAnY(μ-H)(THF)]2 with excess KC8 in the presence of one equivalent of 2.2.2-cryptand (crypt) generates an intensely colored red-brown product crystallographically identified as [K(crypt)][(μ-CpAn)Y(μ-H)]2. The two rings of each CpAn ligand in the reduced anion [(μ-CpAn)Y(μ-H)]21- are attached to two yttrium centers in a "flyover" configuration. The 3.3992(6) and 3.4022(7) Å Y···Y distances between the equivalent metal centers within two crystallographically independent complexes are the shortest Y···Y distances observed to date. Ultraviolet-visible (UV-visible)/near infrared (IR) and electron paramagnetic resonance (EPR) spectroscopy support the presence of Y(II), and theoretical analysis describes the singly occupied molecular orbital (SOMO) as an Y-Y bonding orbital composed of metal 4d orbitals mixed with metallocene ligand orbitals. A dysprosium analogue, [K(18-crown-6)(THF)2][(μ-CpAn)Dy(μ-H)]2, was also synthesized, crystallographically characterized, and studied by variable temperature magnetic susceptibility. The magnetic data are best modeled with the presence of one 4f9 Dy(III) center and one 4f9(5dz2)1 Dy(II) center with no coupling between them. CASSCF calculations are consistent with magnetic measurements supporting the absence of coupling between the Dy centers.

Hydroamination and carboxylative cyclization reactions catalyzed by of gold(I) compounds with 1,1ʹ-bis(phosphino)metallocene ligands

The catalytic activity of [Au2(μ-Cl)(μ-bis(phosphino)ferrocene)][BArF24] (BArF24 = tetrakis(bis(3,5-triflouromethyl)phenyl)borate) compounds was investigated for intermolecular hydroamination reactions and a carboxylative cyclization reaction. Two types of intermolecular hydroamination reactions were studied, the first being the hydroamination of 2-ethynylpyridine with aniline derivatives. Various [Au2(μ-Cl)(μ-bis(phosphino)ferrocene)][BArF24] compounds were successful in catalyzing this reaction, but gave low yields of the desired products. The second type of hydroamination reaction studied was the reaction of phenylacetylene with alkyl amines, including tert‑butylamine, 1-hexylamine, isopropylamine, and cyclohexylamine. While the hydroamination reactions tended to give low yields, the choice of the bis(phosphino)ferrocene ligand did have an impact on the activity of the catalyst. The catalytic activity of these [Au2(μ-Cl)(μ-bis(phosphino)ferrocene)][BArF24] compounds was also studied in the carboxylative cyclization of N-benzyl‑but-2-yn-1-amine. The majority of the gold catalysts examined successfully fixed carbon dioxide to this amine. The effect of changing the bis(phosphino)ferrocene ligands on the catalytic activity of these compounds will be presented for each of these catalytic systems.

Synthesis of disubstituted furans catalysed by [(AuCl)2(μ-bis(phosphino)metallocene)] and Na[BArF24

The catalytic activity of a series of [(AuCl)2(μ-PP)] (PP = 1,1'-bis(phosphino)metallocene ligands) compounds in the presence of Na[BArF24] (BArF24 = tetrakis(3,5-bis(trifluoromethyl)phenyl borate)) was examined in the formation of disubstituted furans from pyridine-N-oxide and terminal alkynes. The products of these reactions were typically the 2,5-disubstituted furans, but in the case of using 2-ethynylpyridene, the 2,4-disubstituted furan formed. The catalytic efficiency was dependent upon both the nature of the terminal alkyne and the 1,1'-bis(phosphino)metallocene ligands. During the course of this study, two new compounds, [(AuCl)2(μ-dppr)] and [(AuCl)2(μ-dppo)] (dppr = 1,1'-bis(diphenylphosphino)ruthenocene; dppo = 1,1'-bis(diphenylphosphino)osmocene), were prepared and characterized by NMR. X-ray crystal structures of both compounds were determined and the oxidative electrochemistry of these new compounds was examined.

Mono-coordinated metallocene ligands endow metal-organic frameworks with highly efficient oxygen evolution and urea electrolysis

Metal-organic frameworks (MOFs) are generally recognized as promising platforms for energy conversion and storage due to their structural diversity and ligands tunability features, yet there are still challenges before practical applications become possible. Herein, a two-dimensional metal-organic framework (MOF) with mono-coordinated ferrocenecarboxylic acid (Fc) is successfully prepared. Different from the pure MOF, the as-prepared Fc-MOF offers remarkable electrocatalytic activity, exhibiting a low overpotential of 273 mV (vs. reversible hydrogen electrode) at 200 mA cm−2 and a small driving potential of 1.49 V (10 mA cm−2) for the overall water splitting with Pt/C as a counterpart cathode. The advance nature of the as-prepared Fc-MOF could also be evidenced by its remarkable Urea oxidation reaction (UOR) activity as it drives a current density of 10 mA cm−2 with a potential of 1.35 V. Theoretical calculations and experimental results demonstrate that the introduction of highly electronegative Fc not only change the electronic structure of MOF but also expose more unsaturated active sites, which in turn varies the adsorption features of the intermediate species during the catalytic process. This work represents a new pathway for exploration of highly efficient MOF based electrocatalysts.

Slow magnetic relaxation in a neodymium metallocene tetraphenylborate complex

Lanthanide-based single-molecule magnet research has historically focused on molecules of the highly anisotropic Dy3+ and Tb3+ ions, while studies on the lighter, more abundant lanthanides are still relatively scarce. Recent advances with Dy3+ metallocene complexes in particular have led to some truly exceptional systems and the highest relaxation barrier to date, in excess of 1200 cm−1. Even still, the design and understanding of systems incorporating the earlier lanthanides is arguably of great importance if the field is to shift in the direction of identifying systems for practical application. Herein, we present magnetic characterization of the mononuclear metallocene complex Cp*2Nd(BPh4) ([Cp*]− = pentamethylcyclopentadienyl anion), which exhibits slow magnetic relaxation with a relaxation barrier of Ueff = 29(1) cm−1 in the presence of a small dc magnetic field, illustrating that the metallocene ligand framework can also be utilized in the design of single-molecule magnets of the earlier lanthanides. We also discuss dynamic magnetic susceptibility data for the Cp*2Ho(BPh4) and Cp*2Er(BPh4) congeners, which reveal that this particular ligand field is best suited to lanthanides ions exhibiting an oblate 4f electron density.

Allylic Alkylations with Enamine Nucleophiles.

Allylic alkylation reactions are some of the most important carbon-carbon bond forming reactions in organic synthesis. In particular, the alkylation of carbonyl substrates provides an efficient pathway to structurally diverse molecules. Such reactions predominantly rely on the in situ generation of a nucleophilic enamine intermediate. This brief personal account describes research carried out in our group concerning the Pd-catalyzed allylic alkylation of carbonyl compounds using these species. Our methodology combines Pd-catalysis with metallocene ligands to carry out these transformations.

Compounds containing weak, non-covalent interactions to the metal in the backbone of 1,1′-bis(phosphino)metallocene ligands

The reactions of Na[BArF] (BArF=tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) with a series of [Pd(dpEMc)Cl2] (dpEMc=1,1′-bis(diphenylphosphino)ruthenocene (dppr), 1,1′-bis(diphenylphosphino)osmocene (dppo) or 1,1′-bis(diphenylarsino)ferrocene (dpaf)) compounds were examined. The reaction of Na[BArF] with [Pd(dppMc)Cl2] (dppMc=dppr or dppo) resulted in the formation of the corresponding chloride-bridged dimers, [Pd(dppMc)(μ-Cl)]22+, which were structurally characterized. However, these dimers appear to be unstable in CH2Cl2 and partially (Ru) or fully (Os) convert into the corresponding monomers, [Pd(dppMc)Cl]+, which are proposed to have a weak, non-covalent M–Pd (M=Ru or Os) interaction. Similar compounds, [Pd(dppo)PPh3]2+ and [Pt(dppMc)PPh3]2+ (M=Fe (1,1′-bis(diphenylphosphino)ferrocene (dppf)) or Os (dppo)), were also prepared and structurally characterized. These compound also exhibit weak, non-covalent M–Pd/Pt interactions. In addition, the electrochemical properties of these compounds were examined by cyclic voltammetry.

Ligand effect in Ti‐mediated living radical styrene polymerizations initiated by epoxide radical ring opening. III. Substituted sandwich metallocenes

AbstractThe ligand effect and reaction conditions for the living radical polymerization of styrene initiated by epoxide radical ring opening were investigated in a series of substituted sandwich metallocenes: L2Ti(IV)Cl2 [L = cyclopentadienyl (Cp), EtCp, iPrCp, tBuCp, pentamethylcyclopentadienyl (Cp*), or indenyl (Ind)], Cp2TiX2 (X = F, Cl, Br, or CO), tBuCp2TiMe2, and PcTiCl2 (Pc = phthalocyanine). The polymerization was mediated by the reversible termination of growing chains with Ti(III) species derived from the Zn reduction of the Ti(IV) precursors. With the exception of Ti derivatives with Cp*, Pc, F, CO, and Me ligands, which could not be reduced, all other complexes catalyzed a living radical process with a linear dependence of the number‐average molecular weight on the conversion and a narrow polydispersity at 70–90 °C. A relatively weak dependence (RCp: R = H ∼ Et ∼ iPr ∼ tBu > Ind) of the polymerization on the nature of the Cp substituent was observed. This was provisionally rationalized as a result of the constant balance between electronic and steric effects with increasing substituent size. By contrast, the apparent Cl > Br ≪ F and L2TiCl2 > LTiCl3 trend qualitatively paralleled the decrease in the Lewis acidity with a decreasing number of Cl ligands and the increased electron donation and steric effects of the metallocene ligands. Thus, sandwich metallocenes are better suited for this polymerization than Ti bisketonates, alkoxides, and half‐sandwich derivatives. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1060–1070, 2006

Metallocenes with Cp Ring-Fused to Chalcogen Heterocycles: Synthesis and Polymerization Results

Substituted metallocene ligands containing cyclopentadiene ring fused to either selenophene or benzo[b]tellurophene were prepared following methods previously developed for analogs containing sulfur. 5-Methyl-4,5-dihydro-cyclopenta[b]-selenophen-6-one ( 2 ) and 3-Methyl-3,4-dihydro-benzo[b]cyclopenta[d]telluraphen-2-one ( 7 ) (major isomers) were prepared by polyphosphoric acid catalyzed Friedel-Crafts acylation/Nazarov cyclization of methacrylic acid onto selenophene and respectively benzo[b]tellurophene. Following reduction of the ketone to alcohol, then dehydration, the chalcogen-containing cyclopentadiene olefins were prepared. The olefins were deprotonated with n-butyllithium followed by either bridging with dichlorodimethylsilane, deprotonation and metallation, or deprotonation and direct metallation. In this manner, isomeric mixtures of –rac/-meso dimethylsilanediylbis(η5-5-Methyl-cyclopenta[b]selenophen-yl)zirconium dichloride ( 5 ) Bis(η5−2-methyl-cyclopenta[b][1]benzotelluraphen-yl)zirconium dichloride ( 9 ) were prepared and characterized. Complexes formed active olefin polymerization catalyst when activated with methylalumoxane. Polymerization results with ethylene and propylene are included.

Comparison of electrochemical behavior of exohedral palladium complexes with [60]- and [70]-fullerenes and metallocene ligands

Electronic structures of exohedral palladium complexes of [60]- and [70]-fullerenes with diphenylphosphinoferrocenyl, diphenylphosphinoruthenocenyl, and diphenylphosphinocymantrenyl ligands were studied by cyclic voltammetry and semiempirical quantum-chemical calculations. Probable sites of localization of electronic changes in the molecules of these complexes under electrochemical oxidation and reduction were determined.

Synthesis of a Novel Ethylene-bis(tetrahydroindenyl) Ligand Containing a Functionalized Four-Carbon Tether.

Abstract In this work we present an efficient synthesis of a novel ethylene-bis(2-methyl-tetrahydroindenyl) ligand, 2b , containing a tethered functional group. The tethered functional group may enable heterogenization of the homogeneous ligand on various solid supports. This strategy is the first attempt of incorporating a functional tether into a bridged metallocene ligand without disrupting the ethane bridge and represents a new approach to heterogenizing metallocene ligands. A practical synthesis was achieved by selective alkylation of the starting material and the Pauson–Khand cyclization. In addition, this strategy offers a new synthetic pathway for the preparation of the ethylene-bis(2-methyl-tetrahydroindenyl) ligand, 1b .

Synthesis of a novel ethylene-bis(tetrahydroindenyl) ligand containing a functionalized four-carbon tether

In this work we present an efficient synthesis of a novel ethylene-bis(2-methyl-tetrahydroindenyl) ligand, 2b , containing a tethered functional group. The tethered functional group may enable heterogenization of the homogeneous ligand on various solid supports. This strategy is the first attempt of incorporating a functional tether into a bridged metallocene ligand without disrupting the ethane bridge and represents a new approach to heterogenizing metallocene ligands. A practical synthesis was achieved by selective alkylation of the starting material and the Pauson–Khand cyclization. In addition, this strategy offers a new synthetic pathway for the preparation of the ethylene-bis(2-methyl-tetrahydroindenyl) ligand, 1b .

Ethene Co- and Terpolymerizations with TIBA-Protected Norbornenemethanol and TIBA-Protected Norbornenecarboxylic Acid Using Homogeneous Metallocene/MAO Catalyst Systems

Ethene copolymerizations were carried out with triisobutylaluminum (TIBA)-protected norbornenemethanol and norbornenecarboxylic acid, respectively, using homogeneous metallocene/MAO catalyst systems. Moreover, ethene terpolymerizations with both polar norbornene derivatives were investigated for the first time. The metallocenes utilized, such as iPr[CpInd]ZrCl2, iPr[(3-iPr-Cp)Ind]ZrCl2, and iPr[(3-tert-But-Cp)Ind]ZrCl2 contain ligand frameworks of various sterical demands. The incorporation of polar monomers into the polymer chain was determined by NMR spectroscopic investigations. Kinetic and analytical results of the polymerization experiments revealed increasing activities and a decreasing comonomer incorporation into the polymer chain with an increasing sterical demand of the metallocene ligand. Additionally, the TIBA-protected norbornenecarboxylic acid shows a lower incorporation rate into the copolymer chain in comparison with the TIBA-protected norbornenemethanol.

Novel Heterobimetallic Compounds with Metal−Metal Bonds: The Use of Quinolyl-Substituted Metallocenes as Tridentate Ligands

1,1‘-Bis(quinolyl)ferrocene (1) and 1,1‘-bis(quinolyl)ruthenocene (2) were prepared by palladium-catalyzed cross-coupling of zincated metallocenes ((C5H4ZnCl)2M; M = Fe, Ru) with 8-bromoquinoline. Both complexes can serve as tridentate ligands toward d10 metal ions. Their reactions with anhydrous zinc chloride or with tetrakis(acetonitrile)copper(I) tetrafluoroborate lead to the bimetallic complexes 3−6, in which the zinc or copper ion is coordinated in a tridentate manner by the metallocene derivative 1 or 2, respectively. Two different coordination modes of the ligands 1 and 2 were observed, both with a metal−metal bond as proved by X-ray analysis. The bond distances Fe−Zn in 3 and Ru−Zn in 4 are practically identical (2.56 A); the Ru−Cu bond in 6 is somewhat longer (2.63 A). Whereas the zinc adducts 3 and 4 contain the metallocene ligand in a Cs-symmetric arrangement with a 4-fold-coordinated Zn atom, the Ru−Cu compound 6 has a C2 symmetry with a 3-fold-coordinated Cu atom. The heterobimetallic compoun...

Ternary metallocene catalyst systems based on metallocene dichlorides and AlBu 3i/[PhNMe 2H][B(C 6F 5) 4] : NMR investigations of the influence of Al/Zr ratios on alkylation and on formation of the precursor of the active metallocene species

The formation of the precursors of the polymerization-active species of the metallocene dichlorides Cp 2ZrCl 2 and Ph 2C(CpFlu)ZrCl 2 by successive reaction with AlBu 3 i and [PhNMe 2H][B(C 6F 5) 4] was investigated by means of NMR spectroscopy. More than two equivalents of AlBu 3 i are required for total conversion of the metallocene dichlorides in the first step. The reaction of Ph 2C(CpFlu)ZrCl 2 with AlBu 3 i leads exclusively to the mono- iso-butyl complex Ph 2C(CpFlu)ZrClBu i, independent of the surplus AlBu 3 i used, whereas in the case of Cp 2ZrCl 2 a series of metallocene products are observed, depending on the Al/Zr ratio used. When this ratio was increased to above 10, the reaction could be exclusively directed to form the dimer metallocene complex [Cp 2ZrH 2·AlBu 3 i] 2. The reaction of [PhNMe 2H][B(C 6F 5) 4] with metallocene/aluminium alkyl mixtures prepared with 10, 20, 50 and 100 equivalents of AlBu 3 i leads to the precipitation of an oily liquid, which contains resulting cationic metallocene complexes. These liquid phases can be purified by extraction and subsequently used for NMR measurements. With one exception, mixtures of two or three different cationic metallocene products are obtained, depending on the Al/Zr ratio and on the metallocene ligand used. An Al/Zr ratio of 100 the reaction of the Ph 2C(CpFlu)ZrCl 2/AlBu 3 i mixture with [PhNMe 2H][B(C 6F 5) 4] exclusively leads to the cationic heterodinuclear metallocene complex [Ph 2C(CpFlu)Zr-μ-H-μ-(C 4H 7)-AlBu 2 i] +, a novel type of allyl-bridged cation, which was characterized by NMR data. None of the reactions of metallocene dichloride/aluminium alkyl mixtures with [PhNMe 2H][B(C 6F 5) 4] lead to the degradation of [B(C 6F 5) 4] −, whereas in the absence of metallocenes AlBu 3 i reacts with [PhNMe 2H][B(C 6F 5) 4] to give AlBu 3− x i(C 6F 5) x compounds. Based on these results and with additional information from the literature a mechanism is proposed to explain the formation of [Ph 2C(CpFlu)Zr-μ-H-μ-(C 4H 7)-AlBu 2 i] +.

Alternating stereospecific copolymerization of ethylene and propylene with metallocene catalysts.

The copolymerization of ethylene and propylene with bridged metallocenes Me(2)E(3-RCp)(Flu)X(2)/MAO (E = C, X = Me; E = Si, X = Cl; R = H or alkyl) was investigated. Ethylene/propylene copolymerization with metallocenes having heterotopic active sites (R =Me, i-Pr) yield alternating, isotactic ethylene/propylene copolymers with percentages of alternating EPE+PEP triads in the range of 61-76% at 50% ethylene incorporation. Both the nature of the substituent R and the bridge E influence the copolymerization behavior including the copolymerization activity, copolymer sequence distribution, molecular weight, and stereochemistry. Silicon-bridged metallocenes produce copolymers with higher activity and molecular weight but lower propylene incorporation at similar feeds than the carbon-bridged analogues. Isotactic PEPEP sequences were observed for all metallocenes, while the tacticities of the EPPE sequences varied with the bridge and the substituent on the metallocene ligand. Isotactic PEPEP sequences and atactic EPPE sequence errors in the alternating copolymers are consistent with a mechanism where the comonomers are enchained alternately at the heterotopic coordination sites of the metallocenes. Isotactic EPPE sequences are indicative of occasional multiple insertions at the stereospecific site, caused by an isomerization of the chain prior to monomer insertion (backskip).