CO2 Complex Research Articles

Kinetic and Thermodynamic Investigations of CO2 Insertion Reactions into Ru–Me and Ru–H Bonds – An Experimental and Computational Study

AbstractThe rates of CO2 insertion into trans‐Ru(dmpe)2(Me)H [1, dmpe = 1,2‐bis(dimethylphosphino)ethane] and trans‐Ru(dmpe)2(Me)2 (2) derivatives were monitored by in situ infrared and 1H NMR spectroscopy. The reactions are first order in both CO2 and metal complex concentrations, and CO2 insertion into the Ru–H bond of 1 occurs instantaneously at 0 °C. The reverse process, decarboxylation, was observed to occur readily at ambient temperature as revealed by 13CO2 exchange with subsequent CO2 insertion into the Ru–Me bond at higher temperatures. No further CO2 insertion into the Ru–H bond of the resulting acetate complex was observed. The activation barrier for CO2 insertion into the first Ru–Me bond of 2 was determined to have ΔH‡ and ΔS‡ values of 12.7 ± 0.6 kcal mol–1 and –31.9 ± 2.0 e.u., respectively, which are indicative of a highly ordered transition state. The rate of CO2 insertion into the second Ru–Me bond was two orders of magnitude slower at ambient temperature and resulted in the formation of trans‐Ru(dmpe)2(O2CMe)2. In general, the insertion of CO2 into the Ru–H or Ru–Me bonds of trans‐Ru(dmpe)2(X)R (R = H or Me) was disvavored in the presence of poorly electron‐donating X ligands. For example, the insertion of CO2 into the Ru–H bond of trans‐Ru(dmpe)2(Cl)H was not observed even under forcing conditions. Computational results were in excellent agreement with these observations and predict a significant enhancement in CO2 activity and resultant complex stability if dmpe is replaced with tetramethylethylenediamine (tmeda).

Quantum chemical studies on adsorption of CO2 on nitrogen-containing molecular segment models of coal

The adsorption of carbon dioxide on N-containing molecular segment models of coal (2-methylpyridine, C13H9N, C23H12N, and N-doped graphenes) has been investigated by the density functional theory including dispersion correction (DFT-D3) method. Four kinds of DFT-D3 methods (BLYP-D3, PBE-D3, BP86-D3 and TPSS-D3 functionals) were used to calculate the binding energy of CO2 with 2-methylpyridine, and the results were examined by benchmark value which was calculated by the coupled-cluster calculation with singles, doubles, and perturbative triple excitations [CCSD(T)] method in the complete basis set (CBS) limit. Due to its best performance, the BLYP-D3 functional was selected to investigate the binding of CO2 to N-doped hydrocarbon clusters (C13H9N and C23H12N). The adsorption of CO2 onto several different adsorption sites and orientations on coal surface models were systematically explored. Our research indicates that increasing the size of the π-system leads to an increase in binding energy. In the C13H9N···CO2 complex, the binding energy is in the range of −2.36–−2.66kcal/mol, while C23H12N···CO2 complex has the results of −3.24–−3.56kcal/mol. We also considered the adsorption of CO2 on the periodic monolayer and bilayer N-doped graphenes. However, as no significant intermolecular charge transfer exists in the physisorption models, the effect of finite ring size on the binding energies in complexes was not obvious.

Novel complexes of Co2+, Ni2+, and Cu2+ with N-(phosphonomethyl)aminosuccinic acid

Complexes of Co2+, Ni2+, and Cu2+ with N-(phosphonomethyl)aminosuccinic acid (H4PMAS) of general formula Na2MPMAS·nH2O [M=Co(II), Ni(II), Cu(II), n—number of water molecules] were synthesized. Based on interpretation of diffusion reflectance spectroscopy, structure of all complexes is based on distorted octahedral. Analysis of IR spectra of Co(II), Ni(II), and Cu(II) N-(phosphonomethyl)aminosuccinates demonstrated that metal ions are coordinated to the ligand through nitrogen atom of the imino group, oxygen atoms of the α- and β-carboxyl groups as well as oxygen atom of the phosphonic group of the H4PMAS. We demonstrated that thermal stability of complexes increases in sequence Cu(II) < Ni(II) < Co(II), obviously as a result of change over from the dimeric to polymeric character of the initial complex. Complete decomposition of ligand occurs at these temperatures and is accompanied by release of H2O, CO2, and NO2. The final products of thermal decomposition of the complexes are mixtures of oxides and phosphates of respective metals.

A theoretical study on the hydrogen bonding interactions in HXeCCH⋯Y (Y = H2O and HF) complexes

The HXeCCH⋯H2O and HXeCCH⋯HF complexes have been investigated by ab initio calculation. Equilibrium geometry, harmonic vibrational frequencies, energies were calculated. The results show that HXeCCH tends to form π H-bond by accepting proton from H2O (or HF) when they encountered, while other complexes taking XeH or CH of HXeCCH moiety as proton donors have less stabilities. The H-bond taking CH as H-donor is characterized to be a red-shifting H-bond, while the H-bonds taking XeH as H-donors in some complexes show blue-shifting character. Meanwhile, the vXeH in π H-bonded complexes shifts to blue although these π H-bonds are red-shifting ones, which is attribute to the hyper-conjugation effects in these complexes. Moreover, the hyper-conjugation effects or the formation of the blue-shifting H-bonds lead to more (HXe)+(CCH)− ionic character, while the HXeCCH⋯H2O complex involving one CH⋯O red-shifting H-bond shows less (HXe)+(CCH)− ionic character. The localized molecular orbital energy decomposition analyses (LMO-EDA) show that the dominant stabilizing forces are exchange energies and electrostatic interactions in all complexes, while the polarization and dispersion interactions play a minor role to the stabilization of these complexes. Compared with paralleled HXeCCH⋯CO2 complex observed experimentally, there are more chances to observe π H-bonded HXeCCH⋯H2O and HXeCCH⋯HF experimentally.

Syntheses and coordination studies of 2-(diphenylphosphinomethyl)pyridine N,P dioxide with Co2+, Ni2+, Cu2+ and Zn2+ tetrafluoroborate

Several new coordination complexes of Co2+, Ni2+, Cu2+ and Zn2+ were synthesized by the reaction of their corresponding tetrafluoroborate salts with 2-(diphenylphosphinomethyl)pyridine N,P dioxide, L. All structures were determined by single-crystal X-ray crystallography showing an octahedral environment around the metal centers. Single crystals of M(BF4)2L2(S)2 (M=Co2+, Ni2+ or Zn2+; S=MeOH or CH3CN) were obtained at −5°C by slow diffusion of ether into solutions of these complexes. When these crystals were re-dissolved in methanol and allowed to grow again at room temperature using the same technique and solvent system, the product obtained presented a 3:1 ratio ligand-to-metal. The complex [Cu(BF4)2L2(MeOH)](BF4), exhibits two units of L, one molecule of MeOH and one counter-anion coordinated to the Cu2+ center. This can be attributed to the steric restrictions generated by the lower symmetry in this complex, a product of significant Jahn–Teller effects, which favors a long range interaction between the metal center and a less coordinative counter-anion, compared to a MeOH unit. A 1:1 ligand-to-metal ratio complex of Ni2+ was also obtained by allowing the starting materials to react for only a short time. Attempts to obtain single crystal of complexes of Co2+, Cu2+ or Zn2+ in a 1:1 ratio were not successful. The resulting 3:1 ligand-to-metal products were fully characterized by elemental analysis, UV–Vis spectroscopy and FT-IR spectroscopy.

2-(1-{2,6-Bis[bis(4-fluorophenyl)methyl]-4-methylphenylimino}ethyl)-6-[1-(arylimino)ethyl]pyridylcobalt dichlorides: Synthesis, characterization and ethylene polymerization behavior

A series of 2-(1-{2,6-bis[bis(4-fluorophenyl)methyl]-4-methylphenyl-imino}ethyl)-6-[1-(arylimino)ethyl]pyridylcobalt(II) chloride complexes (Co1–Co5) was synthesized and characterized by FT-IR spectroscopy and elemental analysis. The molecular structures of the representative cobalt complexes Co1 (R1 = Me, R2 = H), Co4 (R1 = Me, R2 = Me) and Co5 (R1 = Et, R2 = Me) were determined by single crystal X-ray diffraction, and revealed a pseudo-square-pyramidal geometry at cobalt. Upon activation with either MMAO or MAO, all cobalt complex pre-catalysts exhibited high activities [up to the range of 107 g of PE (mol of Co)−1 h−1] toward ethylene polymerization. The influence of reaction parameters and the nature of the ligands were investigated in detail on the catalytic activities of the corresponding complexes and properties of the resultant polyethylene. The cobalt system revealed single-site behavior and produced polyethylene with narrow molecular distribution. Compared to the use of MAO, the use of MMAO produced polyethylene of higher molecular weight.

A comparative investigation of copper and cobalt imprinted polymers: evidence for retention of the solution-state metal ion–ligand complex stoichiometry in the imprinted cavities

A copper imprinted polymer (CuVbMIP) was synthesised using a well characterised 1 : 1 complex of Cu2+ with vinylbenzyliminodiacetic acid (VbIDA). The sorption behaviour of CuVbMIP was compared with that of cobalt imprinted polymer (CoVbMIP) synthesised using the 1 : 2 complex of Co2+ and VbIDA. The complexation properties of the imprinted polymers were investigated using EPR and batch sorption studies. The results indicated that the complexation geometry in the sorption sites were different in the imprinted polymers. The sorption sites generated in the imprinted polymers were found to complex with the metal ions strictly according to the stoichiometry maintained in solution during the preparation of the imprinted polymers.

Adsorption of CO and NO on Ceria- and Pt-Supported TiO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;: &amp;lt;i&amp;gt;In Situ&amp;lt;/i&amp;gt; FTIR Study

Pt/TiO2, Ce/TiO2 and binary system PtCe/TiO2 catalysts were prepared by impregnation method and the structural properties of these catalysts were investigated by means of XRD, CO-TPD and UV-vis diffuse reflectance spectroscopy. As investigated by XRD, the composition of the prepared samples anatase and rutile phases with higher amount of anatase phase and its particle size was in the range of 19 - 22 nm. The band gap also decreased from 3.1 to 2.85 after addition of metal to TiO2. The adsorption and interaction properties of NO and/or CO gases were monitored using an in situ FTIR technique. The intensity and position of the infrared peaks were strongly dependent on the composition of the catalyst. In presence of Pt, the main oxidative reductive products of (NO + CO) are CO2 and NCO complex. The formation of NCO depends on not only the presence of platinum in the catalyst but also the presence of Lewis acid sites which is Ti4+ in this study. However, the interaction between NO and CO gases increased in presence of CeO2. The optimum Ce content in PtCe/TiO2 was 0.1% (Ce/TiO2) at which the maximum peak intensity was observed for NCO and CO2.

A computational study of the influence of the ceria surface termination on the mechanism of CO oxidation of isolated Rh atoms

The reaction mechanism for CO oxidation by isolated Rh atoms stabilized on CeO2(111), CeO2(110) and CeO2(100) surfaces is investigated by a combination of Density Functional Theory and kinetic Monte Carlo calculations. On Rh/ CeO2(111), one adsorbed CO molecule on Rh was found to form a stable intermediate structure with surface O. The reaction cycle cannot be closed because of the strong adsorption of the CO2 complex. The presence of a second adsorbed CO significantly decreases the desorption energy, thus opening a possible reaction path. Formation of the oxygen vacancy is accompanied by reduction of surface cerium. On Rh/CeO2(110), adsorbed CO can easily react with a ceria surface O atom due to the lower Ce-O bond energy. Since surface O atom migration is much more facile on Rh/CeO2(110) than on Rh/CeO2(111), CO2 desorption is also more easy for the former surface. Molecular oxygen will adsorb on the resulting vacancy. After desorption of the second CO2 product molecule by reaction of adsorbed CO with another surface O atom, the adsorbed oxygen molecule migrates spontaneously to the vacancy site and dissociates with negligible barrier. The role of molecular oxygen is to heal the oxygen vacancy rather than being involved in a direct reaction with adsorbed CO. The Rh/ CeO2(100) model was found to be inactive for CO oxidation, mainly because of the geometric constraints for the adsorbed CO molecule to react with one of the surface O atoms, despite the low Ce-O bond energy of the CeO2(100) surface. The main reason is the large distance between the C of adsorbed CO and the ceria O surface atoms. The particularities of the CO oxidation mechanism for isolated Rh atoms on these ceria surfaces are in agreement with the experimental activity trends.

Guest-Induced Crystal-to-Crystal Transitions of Poly(l-lactide) Complexes

In this study, we systematically investigated various crystal-to-crystal transitions relating to poly(L-lactide) (PLLA) cocrystallized with low-molecular-weight compounds using wide-angle X-ray diffraction and Fourier transform infrared spectroscopy. First, the solvent exchange and the resultant crystal transition of solvent complexes were investigated. Basically, the solvent exchange treatments at -25 °C became successful, although some specific phenomena depending on solvent species were seen. The ratio of the α-form in the crystalline region increased by an increase in the solvent exchange temperature. Second, the crystal transition behavior between CO(2) and solvent complexes was investigated. The complete transition from solvent complex to CO(2) complex was observed for all solvent complexes. In contrast, it was found that types of solvents and the surrounding temperature have a great effect on the transition behavior from CO(2) to solvent complexes. Finally, the guest-induced transitions of noncomplex crystals (α-, α'-, and α″-forms) were examined. As a result, it was revealed that the guest-induced transition behavior of noncomplex crystals was much influenced by the order of crystal (chain conformation and packing) of noncomplexes (α, α', and α″) as well as kinds of guests.

Analysis of Co–Pd alloys deposition from electrolytes based on [Co(NH3)6]3+ and [Pd(NH3)4]2+ complexes

This work comprises results of the studies on the mechanism and kinetics of hexaamminecobalt(III) and tetraamminepalladium(II) co-reduction in ammonia–chloride solutions at pH 9.5. Electrolyte stability was determined by UV–Vis absorbance measurements. Electrochemical investigations were performed by applying cyclic voltammetry (CV) combined with electrochemical quartz crystal microbalance (EQCM) as well as with spectrophotometric measurements. The results indicated that the reduction process of [Co(NH3)6]3+ consists of two steps. In the first step [Co(NH3)6]3+ is reduced to [Co(NH3)4]2+ and [Co(NH3)5]2+, and then ammonia complexes of Co2+ are reduced to Co0. The M/z values determined during electrogravimetric measurements indicate that the reduction process of [Pd(NH3)4]2+ takes place in one step due to 2-electrons irreversible reduction reaction. Deposition of Co and Pd take place only in the overpotential deposition range. It was found that the examined reactions are diffusion-controlled. In anodic part M/z values and registered CV's indicate cobalt(II) hydroxide oxidation.

Infrared diode laser spectroscopy of H2O–CO2 and D2O–CO2 complexes in the v2 bend region of water

The rovibrational spectrum of H2O–CO2 and D2O–CO2 weakly bound complexes has been measured in the v2 bend region of the water monomer by infrared diode laser absorption spectroscopy. Rotational and centrifugal distortion constants for different internal rotation states in the excited vibrational state have been determined. The band-origin of the spectrum is blue-shifted from that of the water monomer by 0.739cm−1 for H2O–CO2 and 0.083cm−1 for D2O–CO2, respectively. The tunneling splitting is reduced upon vibrational excitation by 0.00698(31)cm−1 for H2O–CO2 and 0.00044(17)cm−1 for D2O–CO2, respectively.

POTENTIAL ENERGY SURFACES AND MICROWAVE SPECTRA FOR 20Ne–13C16O2, 22Ne–12C16O2 and 22Ne–13C16O2 COMPLEXES

We report averaged potential energy surfaces for isotopic Ne–CO2 complexes (20 Ne –13 C 16 O 2, 22 Ne –12 C 16 O 2 and 22 Ne –13 C 16 O 2). According to the latest ab initio potential of 20 Ne –12 C 16 O 2 (Chen R, Jiao EQ, Zhu H, Xie DQ, J Chem Phys133:104302, 2010) including the Q3 normal mode for the υ3 antisymmetric stretching vibration of the CO2 molecule. We obtain the averaged potentials for 20 Ne –13 C 16 O 2, 22 Ne –12 C 16 O 2 and 22 Ne –13 C 16 O 2 by the integration of the three-dimensional potential over the Q3 coordinate. The averaged potential surfaces are found to have a T-shaped global minimum and two equivalent linear local minima. The radial DVR/angular FBR method and the Lanczos algorithm are applied to calculate the rovibrational energy levels. Comparison with the available observed values showed an overall excellent agreement for all spectroscopic parameters and the microwave spectra.

Computational Studies of CO2 Activation via Photochemical Reactions with Reduced Sulfur Compounds

Reactions between CO(2) and reduced sulfur compounds (RSC), H(2)S and CH(3)SH, were investigated using ground and excited state density functional theory (DFT) and coupled cluster (CC) methods to explore possible RSC oxidation mechanisms and CO(2) activation mechanisms in the atmospheric environment. Ground electronic state calculations at the CR-CC(2,3)/6-311+G(2df,2p)//CAM-B3LYP/6-311+G(2df,2p) level show proton transfer as a limiting step in the reduction of CO(2) with activation energies of 49.64 and 47.70 kcal/mol, respectively, for H(2)S and CH(3)SH. On the first excited state surface, CR-EOMCC(2,3)/6-311+G(2df,2p)//CAM-B3LYP/6-311+G(2df,2p) calculations reveal that energies of <250 nm are needed to form H(2)S-CO(2) and CH(3)SH-CO(2) complexes allowing facile hydrogen atom transfer. Once excited, all reaction intermediates and transition states are downhill energetically showing either C-H or C-S bond formation in the excited state whereas only C-S bond formation was found in the ground state. Environmental implications of these data are discussed with a focus on tropospheric reactions between CO(2) and RSC, as well as potential for carbon sequestration using photocatalysis.

Electrodeposition of Crystalline Co3O4—A Catalyst for the Oxygen Evolution Reaction

Crystalline films of Co3O4 are deposited by electrochemically oxidizing a tartrate complex of Co2+ in an aqueous, alkaline solution at elevated temperatures. The crystallinity and stability of the films are a strong function of the deposition temperature. Films deposited at temperatures from 50 to 90 °C are amorphous, but films deposited from refluxing solution at 103 °C are crystalline. The crystalline films adhere strongly to the substrate, whereas the amorphous films peel off of the substrate when dried due to drying stresses. The crystalline films deposit with the normal spinel structure, with a lattice parameter of 0.8097 nm and crystallite size of 26 nm. The catalytic activity of Co3O4 for the oxygen evolution reaction (OER) of the crystalline and amorphous films is compared by Tafel analysis in alkaline solution at pH 14. The crystalline Co3O4 film has a Tafel slope of 49 mV/decade and an exchange current density of 2.0 × 10–10 A cm–2, whereas an amorphous film deposited at 50 °C has a Tafel slope ...

Experimental and Theoretical Investigation of Homogeneous Gaseous Reaction of CO2(g) + nH2O(g) + nNH3(g) → Products (n = 1, 2)

Decreasing CO2 emissions into the atmosphere is key for reducing global warming. To facilitate the CO2 emission reduction efforts, our laboratory conducted experimental and theoretical investigations of the homogeneous gaseous reaction of CO2(g) + nH2O(g) + nNH3(g) → (NH4)HCO3(s)/(NH4)2CO3(s) (n = 1 and 2) using Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy and ab initio molecular orbital theory. Our FTIR-ATR experimental results indicate that (NH4)2CO3(s) and (NH4)HCO3(s) are formed as aerosol particulate matter when carbon dioxide reacts with ammonia and water in the gaseous phase at room temperature. Ab initio study of this chemical system suggested that the reaction may proceed through formation of NH3·H2O(g), NH3·CO2(g), and CO2·H2O(g) complexes. Subsequent complexes, NH3·H2O·CO2 and (NH3)2·H2O·CO2, can be formed by adding gaseous reactants to the NH3·H2O(g), NH3·CO2(g), and CO2·H2O(g) complexes, respectively. The NH3·H2O·CO2 and (NH3)2·H2O·CO2 complexes can then be rearranged to produce (NH4)HCO3 and (NH4)2CO3 as final products via a transition state, and the NH3 molecule acts as a medium accepting and donating hydrogen atoms in the rearrangement process. Our computational results also reveal that the presence of an additional water molecule can reduce the activation energy of the rearrangement process. The high activation energy predicted in the present work suggests that the reaction is kinetically not favored, and our experimental observation of (NH4)HCO3(s) and (NH4)2CO3(s) may be attributed to the high concentrations of reactants increasing the reaction rate of the title reactions in the reactor.

EQCM, SEC and voltammetric study of kinetics and mechanism of hexaamminecobalt(III) electro-reduction onto gold electrode

The work comprises results of the studies on the mechanism and kinetics of hexaamminecobalt(III) reduction in ammonia-chloride solutions at pH 9.5. The studies were preceded by the thermodynamic analysis of the electrolyte. Electrolyte stability was determined by UV–vis absorbance measurements. Electrochemical examinations were performed by applying cyclic voltammetry (CV) combined with Electrochemical Quartz Crystal Microbalance (EQCM) as well as with spectrophotometric measurements. The results indicated that reduction process of [Co(NH3)6]3+ consists of two steps. In the first step [Co(NH3)6]3+ is reduced to [Co(NH3)4]2+ and [Co(NH3)5]2+, and then ammonium complexes of Co2+ are reduced to Co0. Deposition of Co takes place only in the overpotential deposition range. Values of electrochemical equivalents M/z within this range indicate formation of Co(OH)2 phase. The examined reactions are diffusion controlled. Diffusion coefficients of complexes participating in particular reactions were determined with the use of Randles–Sevcik and Berzins–Delahay equations. In anodic part M/z values indicate 2-electrons reaction of cobalt oxidation connected with chemical dissolution of cobalt hydroxide formed during cathodic sweep.

Crystal structures of poly(l-lactide)–CO2 complex and its emptied form

In this study, the crystal polymorph of poly(l-lactide) (PLLA) formed under high-pressure CO2 and its transition behavior with CO2 desorption were examined using mainly wide-angle X-ray diffraction and Fourier transform infrared spectroscopy. We demonstrated that PLLA forms the complex crystal with CO2 under high-pressure CO2 below room temperature, and the crystal transition to the α-form, which is accompanied with gradual changes in the packing and conformation of PLLA chains, occurs with CO2 desorption and subsequent annealing in air (annealing was needed only for films with a draw ratio smaller than three). Compared with the α-form, the oriented CO2 complex film showed shorter a-, longer b-, and shorter c-axis lengths, resulting in a slight increase in the unit cell volume. The hexagonal packing (a/b ≈ 1.73), which is seen in the α-form, no longer exists for the oriented CO2 complex film (a/b ≈ 1.33). It was indicated that the chain helical conformation of PLLA in the CO2 complex is similar but different to that in the α-form (10/7 helix for both forms), because of the interactions between PLLA and CO2. It seems likely that CO2 molecules are encapsulated in the cavity surrounded by four PLLA chains. With CO2 desorption, the a-axis length increased and the b-axis one decreased, so that a/b increased to 31/2 (hexagonal packing), keeping the orthorhombic system. It was proposed that the formation of α″-crystals results from PLLA being trapped in the quasi-stable state during the CO2 complex-to-α-form transition, and the energy barrier between the α″- and α-forms can be overcome by only CO2 desorption in the case of a draw ratio higher than two.

Bimetallic poly- and oligo-nuclear complexes based on a rhodium(III) metalloligand

Interaction of trans-[Rh(i-Nic)4Cl2]3− anions (i-Nic− – isonicotinate anion) with Cuaq2+ and Coaq2+ cations in water has afforded complex salts Co3[Rh(i-Nic)4Cl2]2·17H2O (1) and Cu3[Rh(i-Nic)4Cl2]2·14H2O (2). Oligonuclear character of 1 and 2 has been established. A coordination polymer Cu5[Rh(i-Nic)4Cl2]2(i-Nic)2(OH)2·2H2O (3) has been crystallized by hydrothermal treatment of 2 at 160°C, and its structure was determined by X-ray structural analysis. EPR data for the complexes has been collected and interpreted. Thermal decomposition of the salts was studied by c-DTA. Bimetallic alloys rhodium–copper and rhodium–cobalt have been obtained as final products of thermal decomposition.

2-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]-6-[1-aryliminoethyl]pyridyl cobalt dichlorides: Synthesis, characterization and ethylene polymerization behavior

A series of 2-[1-(2,6-dibenzhydryl-4-chlorophenylimino)ethyl]-6-[1-aryliminoethyl]pyridyl cobalt(II) dichloride complexes (Co1–Co5) were prepared and characterized by elemental and spectroscopic analysis. The molecular structures of the representative cobalt complexes Co1 and Co4 were determined by single-crystal X-ray diffraction, and revealed a pseudo-square-pyramidal geometry around each cobalt atom. Upon activation with either MMAO or MAO, all cobalt pre-catalysts exhibited high activities toward ethylene polymerization and produced polyethylene products with narrow molecular distributions (1.98–3.61) and molecular weights in the range 72.3–665 kg/mol; the latter is higher than any other molecular weight of polyethylenes obtained by previous cobalt pre-catalysts. Reaction parameters and the nature of the ligands were investigated for their influence on the resultant catalytic activities and properties of the polyethylene products obtained.