Relative Lewis Acidity Research Articles

CO2 reduction via oxidative dehydrogenation and dry reforming of ethane over Fe3Ni1 nanoparticles: The influence of the oxide support

The effect of the CO2:C2H6 feed ratio, the relative Lewis acidity of reducible and unreducible catalytically active metal oxide supports with and without Fe3Ni1 alloy nanoparticles on the activity, selectivity and stability for the CO2-mediated oxidative dehydrogenation of ethane (CO2-ODHE) is investigated. To circumvent the influence of the typically dissimilar textural properties of the supports in bulk form, overlayer oxide supports of V, Cr, Ga, Ti or Sm coated on a common γ-Al2O3 carrier were employed. Separately, (Ni0.75Fe0.25)Fe2O4 precursor nanoparticles were synthesized via a nonaqueous surfactant-free method, sonication-deposited onto supports and reduced in situ into an Fe3Ni1 alloy microstructure of bcc and fcc mixed phases captured with in situ XRD. When exposed to carbon dioxide at 255 °C, a selective re-oxidation of the bcc phase via CO2 dissociation is observed, while the fcc phase stays stable and only partially re-oxidizes above 525 °C. Upon exposure to CO2-ODHE conditions, the initial activity of the bare supports increases with increasing acid site strength, but this activity is rapidly lost in case of the strongly acidic supports. Comparison of the C2H4 and CO selectivity indicate direct dehydrogenation is preferred over the oxidative dehydrogenation pathway and is initially occurring in combination with some CO-forming routes, possibly the dry reforming of C2H6. This CO forming route is significant over the most acidic and reducible VOx@Al2O3 support in the early stages of operation. The addition of the Fe3Ni alloy increases the conversions of both C2H6 and CO2 across all supports, with a notably stronger effect observed on CO2 conversion especially over the highly acidic and reducible VOx@Al2O3 and CrOx@Al2O3. As a result, the CO selectivity is increased due to ethane dry reforming activity over the latter supports while CO2-ODHE activity is observed over the supports with intermediate and weak acid sites.

Enhancing CO2 Capture via Metal-Ligand Cooperativity: Tuning Ligand Basicity and Zn(II) Lewis Acidity.

A series of thiosemicarbazonato-hydrazinatopyridine zinc(II) complexes were evaluated as direct air CO2 capture agents. The complexes sequester CO2 in a methanol solution as a metal-coordinated methylcarbonate. The reaction is reversible upon sparging of solutions with an inert gas (N2 or Ar). The capture process involves metal-ligand cooperativity with the noncoordinating nitrogen of the hydrazinatopyridine functional group serving as a Brønsted-Lowry base and the zinc acting as a Lewis acid. In this study, the pendent amine of the thiosemicarbazonato group was varied to include 4-phenyl (ZnL5), 4-(trifluoromethyl)phenyl (ZnL6), 4-cyanophenyl (ZnL7), 4-tolyl (ZnL8), and 4-naphthyl (ZnL9). Hyperconjugation between the pendent group and the ligand core resulted in modulation of the metal ion acidity, as quantified by ligand exchange equilibrium constants (K3 = 193-511) and ligand basicity (pKa,MeOH = 11.09-11.94). Variations in electronic structure that decreased ligand basicity were more than offset by increases in Lewis acidity. The equilibrium constant (K1) for CO2 capture varied from 46300 to 73700. Overall, the value of K1 was directly related to the relative Lewis acidity of the complexes (K3). Notably, there was an overall inverse relationship between K1 and the ligand basicity. The results provide insights into ligand design to further improve CO2 capture.

Theoretical study of fructose adsorption and conversion to trioses on metal-organic frameworks.

The conversion of chemically modified biomass into more valuable chemicals has recently gained significant attention from industry. In this study, we investigate the adsorption of fructose and its conversion into two trioses, glyceraldehyde (GLA) and dihydroxyacetone (DHA), on metal-organic frameworks using density functional theory calculations. The reaction mechanism proceeds through two main steps: first, the opening of the fructose ring; second, the retro-aldol fragmentation, which is favored over intramolecular hydrogen shifts. The substitution of a tetravalent metal in the metal-organic framework leads to different adsorption strengths in the order Hf-NU-1000 > Zr-NU-1000 > Ti-NU-1000. The catalytic activities of Hf-NU-1000 and Zr-NU-1000 are found to be similar. Both are more active than Ti-NU1000, corresponding to their relative Lewis acidity. It was found that functionalization of the organic linkers of the Hf-NU-1000 MOF does not improve its catalytic activity. The catalytic activity follows the order Hf-MOF-808 > Hf-NU-1000 > Hf-UIO-66 when based on either the overall activation energy or the turnover frequency (TOF).

Analyzing Fluoride Binding by Group 15 Lewis Acids: Pnictogen Bonding in the Pentavalent State.

We report the results of a computational investigation into fluoride binding by a series of pentavalent pnictogen Lewis acids: pnictogen pentahalides (PnX5), tetraphenyl pnictogeniums (PnPh4+), and triphenyl pnictogen tetrachlorocatecholates (PnPh3Cat). Activation strain and energy decomposition analyses of the Lewis adducts not only clearly delineate the electrostatic and orbital contributions to these acid-base interactions but also highlight the importance of Pauli repulsion and molecular flexibility in determining relative Lewis acidity among the pnictogens.

Theoretical Quantification of the Lewis Acidity of N‐Heterocyclic Iodonium Salts

AbstractThe quantitative Lewis acidity of N‐heterocyclic iod(az)olium salts is helpful for their further application in organic chemistry. The acidity scales of different N‐Heterocyclic iodonium salts (NHISs) in acetonitrile are investigated using Density functional theory (DFT) studies. There are a total of 121N‐Heterocyclic iodonium salts that have been designed by combining N‐heterocyclic and iodonium salt structures in this research. Theoretical calculations systematically established the 31P NMR chemical shifts of NHISs, and the relative Lewis acidity scale was derived. The Lewis acidity trend is analyzed and explained by the natural population analysis (NPA) charges, molecular electrostatic potential, and the interaction between the iodonium center and the N‐heterocyclic substituents. This study is expected to provide a practical quantitative theoretical scale for the understanding and design of hypervalent iodine(III) reagents with versatile Lewis acidity.

Study of the Cycloaddition of CO2 with Styrene Oxide Over Six-Connected spn Topology MOFs (Zr, Hf) at Room Temperature.

A series of MOFs with a 6-connected spn topology were synthesized (MOF-808-(Zr, Hf), PCN-777-(Zr, Hf), MOF-818-(Zr, Hf)). Through the in situ DRIFTS of NH3 adsorption-desorption, we found that the activated catalyst mainly contains Lewis acid sites. The effects of different organic ligands on the Lewis acid of the Zr6 cluster were analyzed by XPS and NH3 -TPD, and the relative Lewis acidity of the same metal was obtained: PCN-777>MOF-808>MOF-818. In the Py-FTIR results, we confirmed that MOF-818 has a higher acid site density. In the activity test, MOFs with mesoporous structure showed better catalytic activity under normal temperature and pressure. Among them, MOF-818 can still maintain a high degree of crystallinity after catalysis. Finally, we use density functional theory to propose the mechanism of the cycloaddition reaction of carbon dioxide and styrene oxide. The results show that the metal is coordinated with styrene oxide and halogens attack the β carbon of the epoxide.

Fluorescent Lewis Adducts: A Practical Guide to Relative Lewis Acidity

Experimentally determining the strength of a Lewis acid is a highly desirable and important task that has implications across the chemical sciences. Recently, we developed a new fluorescence-based ...

Investigation of Lewis Acid-Carbonyl Solution Interactions via Infrared-Monitored Titration.

Lewis acid-activation of carbonyl-containing substrates is broadly utilized in organic synthesis. In order to facilitate the development of novel reaction pathways and understand existing methods, it is necessary to determine the solution interactions between Lewis acids and Lewis bases. Herein, we report the application of in situ infrared spectroscopy and solution conductivity toward the identification of the solution structures formed when a range of carbonyl compounds are combined with catalytically active metal halide Lewis acids under synthetically relevant conditions. These data are consistent with formation of Lewis acid-dependent complexes, where metals of low relative Lewis acidity display no ground state interaction with carbonyls. Conversely, we observed the formation of polyligated complexes when stronger Lewis acids (SnCl4, TiCl4, ZrCl4, FeCl3, and AlCl3) were treated with ketones, aldehydes, and esters. This collection of observations is intended to assist the synthetic chemist in the design of new catalysts and the development of novel methods.

NMR Quantification of the Effects of Ligands and Counterions on Lewis Acid Catalysis.

The relative Lewis acidity of a variety of metal-ligand catalyst complexes is quantified using 31P NMR spectroscopy. Three 31P NMR probes, including two new bidentate binding probes, are compared on the basis of different binding modes (i.e., monodentate vs bidentate) and the relative scale of their downfield shift upon binding to Lewis acid complexes. Bidentate coordination of catalyst complexes including metal catalysts, ligands, and counterions were assessed due to their importance to asymmetric catalysis. The effect of ligands, counterions, and additives on Lewis acidity is quantified and correlated to reaction yield at an early time point as an approximation for catalytic activity/efficiency and chelation mode in two organic transformations. Binding studies were performed under catalytically relevant conditions, giving further applicability to synthesis. Insight into activation modes are revealed through this analysis.

The Effect of Al2O3 ALD Precursors on the Electrochemical Performance of Cathode Materials for Lithium-Based Battery Applications

Al2O3 coatings prepared by atomic layer deposition (ALD) using trimethyl aluminum (TMA) and H2O on various cathode materials have been shown to enhance the battery cycle stability of lithium ion batteries. Interestingly, not all cathode materials are enhanced by Al2O3 ALD coating. For example, while Al2O3 coating on lithium cobalt oxide (LCO) using 1-2 TMA/H2O ALD cycles dramatically improves cycle performance [1], much less positive effect on lithium manganese oxide (LMO) was observed [2]. We revealed the TMA surface reactions on LMO are unusual in that they do not involve hydroxyls, ethane is released, and the manganese undergoes redox chemistry from our previous study [2]. Furthermore, density functional theory (DFT) calculations reveal that this unique mechanism is driven by the large free energy changes upon methyl loss from TMA [2]. This information leads us to speculate that the surface reactions and subsequent electrochemistry might also depend on different Al precursors. To evaluate our hypothesis, we are exploring a range of aluminum precursors including trimethyl aluminum (TMA), tris(dimethylamido) aluminum (Al-TDMA), aluminum trichloride (AlCl3), dimethyl aluminum isopropoxide (DMAI), and aluminum triisopropoxide (ATIP) on LCO and LMO cathode materials. Our preliminary results showed that very different ALD chemistries on between LCO and LMO cathode materials. It suggests a correlation between cation reduction on the cathode surface and the relative Lewis acidity of the Al precursor ligands. We will elaborate on these findings using results from XPS measurements, DFT calculations, electrochemical characterizations, and battery cycling studies. [1] Y. S. Jung et. al., J. Electrochem. Soc. 157 (1) A75-A81, (2010). [2] L. Chen et. al., Chem 4 2418-2435 (2018).

Understanding the Effects of Ultra-Thin Al2O3 Coatings Prepared By Atomic Layer Deposition on Lithium Ion Battery Cathode Materials

Although various cathode materials possessing high capacity have been developed to date, most of them are still suffering from early capacity fading. Several strategies such as doping, surface treatment and coating have been adopted to minimize this problem. Among a variety of coating strategies, atomic layer deposition (ALD) is regarded as the best technique for coating of cathode materials. ALD is a thin-film growth technique that offers the unique capability to coat complex, three-dimensional objects with precise, conformal layers by atomic-level control over thickness and composition. Over the last decade, ALD has emerged as an effective strategy for coating cathode materials to enhance the overall performance Li-ion batteries. ALD Al2O3 coatings prepared using trimethyl aluminum (TMA) and H2O have been evaluated previously on cathodes including LiCoO2 (LCO), LiMn2O4 (LMO), and Li(NixMnyCoz)O2 (NMC), and the effect of the coating on cycling stability was found to vary between these different materials. Moreover, our recent study of TMA/H2O chemistry on LMO revealed a unique nucleation mechanism leading to unexpected products and altered electrochemical behavior. In an effort to better understand these observations, we are exploring Al2O3 ALD using a range of aluminum precursors (TMA, tris(dimethylamido) aluminum, aluminum trichloride, dimethyl aluminum isopropoxide, and aluminum triisopropoxide) and a variety of cathode materials (LCO, LMO, and several NMCs). We discovered that the ALD aluminum precursor reduces cations on the cathode surface, and the degree of reduction depends on the relative Lewis acidity of the precursor ligands, and on the composition of the cathode surface. In this presentation we will elaborate on these findings and describe the correlations between the chemistry of the ALD-modified cathode surface and the resulting electrochemical behavior and cycling performance.

First Principles Studies of Solid-Solid Interfaces between LiMn2O4 and Protective Coatings

Numerous studies have shown that protective surface coatings are effective in reducing the well-documented problem of Mn ion dissolution from the surface of the spinel LiMn2O4 (LMO) Li-ion battery cathode.[1] Although recent computational work has identified many promising coating chemistries based on bulk thermochemical properties,[2,3] there remains much to be understood regarding the interface formed between protective coatings and the LMO electrode and how such interfacial properties may influence electrochemical performance. In this work, first principles density functional theory calculations are performed on models for both oxide and metallic coatings on the LMO surface. First, we elucidate the mechanism for the onset of Al2O3 film growth by atomic layer deposition (ALD) and show that precursor decomposition in early ALD pulses leads to adsorption site blocking and sub-monolayer film growth.[4] During ALD, the trimethylaluminum [Al(CH3)3] precursor loses its -CH3 groups to the LMO surface, where the Al(CH3)x* and CH3* surface adsorbates each behave as Lewis bases (electron donors), where near-surface Mn ions are the Lewis acidic adduct (electron acceptor). These studies are extended to various thermodynamically stable low- and high-index LMO surface models,[5] demonstrating that ALD on LMO is structure-sensitive toward stepped sites, which are more electron accepting as suggested by calculated oxygen vacancy formation energies. Moreover, we find that the thermodynamics of decomposed intermediates are linearly related to the corresponding oxygen vacancy formation energy, or the relative Lewis acidity of near-surface Mn ions, of each given surface facet and termination. We propose that selective ALD leads to partial coatings formed at defect sites which may be particularly vulnerable to Mn dissolution. Next, we model the process of delithiation near the surface of an LMO coated with metallic Au, which effectively mitigates Mn dissolution. Moreover, Au coatings have been shown to improve kinetics and do not strip under the potentials where LMO electrochemistry occurs.[6] DFT calculations show that a Li+-deficient near-surface is thermodynamically favored for Au-coated LMO. We demonstrate through local density of states and chemical bonding analyses that Au hybridizes with LMO upon delithiation, leading to partial oxidation on Au electronic states rather than full localization of holes on Mn ions. The Au coating has a lower work function than LMO, driving electron transfer from Au to LMO. This is also associated with downward bending of the LMO conduction band in response to Fermi level pinning at the LMO/Au interface. Operando X-ray diffraction experiments show that these interfacial effects may translate to a reduction in the amount of Li+ that can intercalate into the bulk of Au-coated LMO. These studies help elucidate the structure-property relationships of protective coatings at the LMO surface. In particular, this work highlights the driving forces for nucleation and growth of protective coatings by ALD, as well as the influence of interfacial electronic structure on intercalation chemistry of coated electrode materials. This research is supported as part of the Center for Electrochemical Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Effects of Lewis acidity of metal oxide promoters on the activity and selectivity of Co-based Fischer–Tropsch synthesis catalysts

Metal oxides of Ce, Gd, La, Mn, and Zr were investigated as promoters for improving the activity and selectivity of Co-based FTS catalysts. The extent to which these promoters decrease the selectivity toward CH4 and increase the selectivity toward C5+ hydrocarbons was found to depend on both the loading and the composition of the oxide promoter. Elemental mapping by STEM–EDS revealed that the propensity for a given metal oxide to associate with Co affects the sensitivity of the product distribution to changes in promoter loading. For all promoters, a sufficiently high loading resulted in the product distributions becoming insensitive to further increases in promoter loading, very likely due to the formation of a half monolayer of promoter oxide over the Co surface. Simulations suggest that the fraction of Co active sites that are adjacent to the promoter moieties approaches unity at this degree of coverage. The oxidation state of the promoter metal cation under reaction conditions, determined by in situ XANES measurements, was used to calculate relative Lewis acidity of the promoter metal cation. A strong positive correlation was found between the C5+ product selectivity and the Lewis acidity of the promoter metal cations, suggesting that the promotional effects are a consequence of Lewis acid–base interactions between the reaction intermediates and the promoter metal cations. Rate data obtained at different pressures were used to estimate the apparent rate coefficient and the CO adsorption constant appearing in the Langmuir–Hinshelwood expression that describes the CO consumption kinetics for both unpromoted and the metal oxide-promoted catalysts. Both parameters exhibited positive correlations with the promoter Lewis acidity. These results are consistent with the hypothesis that the metal cations of the promoter act as Lewis acids that interact with the O atom of adsorbed CO to facilitate CO adsorption and dissociation.

Preferential Solvation of Lithium Cations and Impacts on Oxygen Reduction in Lithium-Air Batteries.

The solvation of Li+ with 11 nonaqueous solvents commonly used as electrolytes for lithium batteries was studied. The solvation preferences of different solvents were compared by means of electrospray mass spectrometry and collision-induced dissociation. The relative strength of the solvent for the solvation of Li+ was determined. The Lewis acidity of the solvated Li+ cations was determined by the preferential solvation of the solvent in the solvation shell. The kinetics of the catalytic disproportionation of the O2•- depends on the relative Lewis acidity of the solvated Li+ ion. The impact of the solvated Li+ cation on the O2 redox reaction was also investigated.

Efficient acetalization of benzaldehydes using UiO-66 and UiO-67: Substrates accessibility or Lewis acidity of zirconium

Porous metal-organic frameworks of Zr6O4(OH)4 clusters connected by organic linkers of 1,4-benzenedicarboxylate (UiO-66) and biphenyl-4,4′-dicarboxylate (UiO-67) were prepared using a solvothermal method and used as efficient catalysts for acetalization of benzaldehydes with alcohols. UiO-66 showed better catalytic activity than UiO-67, (i.e. 91% vs 86%) when the catalyst was employed in the benzaldehyde acetalization in methanol for an hour at r.t. (28°C). UiO-66 was further explored as the catalyst to observe effects of catalyst concentrations, alcohols, bulkier benzaldehydes, and recyclability. The performance of other catalysts, including the MOF precursor, was also examined to compare the activity and to explain the importance of a MOF structure. Relative Lewis acidity of Zr in UiO-66 and UiO-67 was achieved by natural bond orbital (NBO) analysis using DFT/B3LYP method. It was found that accessibility of substrates to internal active sites might play a dominant role than the Lewis acidity of Zr-MOFs.

Colloidal Indium-Doped Zinc Oxide Nanocrystals with Tunable Work Function: Rational Synthesis and Optoelectronic Applications

Transition metal oxides are widely used in solution-processed optoelectronic devices as charge-transporting interlayers to improve contact properties and device performances. Here we show that the work function of oxide nanocrystal thin films, one of the most important parameters for charge-transporting interlayers, is readily tuned by rational design of material synthesis. Mechanism studies reveal that the combination of employing the reverse-injection approach and using zinc stearate and indium 2-ethylhexanoate as the cationic precursors ensures both controlled reaction pathways and balanced relative dopant-host precursor reactivity and hence high-quality indium doped zinc oxide nanocrystals. We find that the empirical rule of relative Lewis acidity fails to predict the relative reactivity of the cationic precursors and quantitative measurements are obligatory. The successful incorporation of indium dopants into host oxide nanocrystals accompanied by the generation of high density of free electrons leads to oxide thin films with lower work function. Polymer light-emitting diodes with electron-transporting interlayers based on the indium doped zinc oxide nanocrystals exhibit improved electron-injection properties and enhanced device characteristics, i.e., lower turn-on voltage, higher maximum luminance, and higher efficiency. Our study is an excellent example that new understanding on the chemical kinetics of doped nanocrystals leads to rational design of synthetic protocols and materials with tailored electronic properties, providing benefits for their optoelectronic applications.

Heterolytic activation of hydrogen using frustrated Lewis pairs containing tris(2,2′,2′′-perfluorobiphenyl)borane

The extremely sterically hindered borane tris(2,2',2''-perfluorobiphenyl)borane (PBB) has been structurally characterised. In combination with bulky nitrogen bases, it forms the 'frustrated Lewis pairs' (FLPs) PBB/2,2,6,6-tetramethylpiperidine (TMP) (1), PBB/1,4-diazobicyclo[2.2.2]-octane (DABCO) (2) and PBB/2,6-lutidine (lut) (3). These novel, unquenched acid-base pairs have been shown to effect facile room temperature heterolytic cleavage of dihydrogen to form the ammonium borate salts [2,2,6,6-Me(4)C(5)H(6)NH(2)][HB(C(12)F(9))(3)] (4) and [N(C(2)H(4))(3)NH][HB(C(12)F(9))(3)] (5), and lutidinium borate [2,6-Me(2)C(5)H(3)NH][HB(C(12)F(9))(3)] (6). Although these reactions are equilibria, the reverse reaction and release of hydrogen gas was not apparent at temperatures up to 120 °C. The relative Lewis acidity of PBB has been determined using the Gutmann-Beckett method.

NHC-Derived Bis(amidiniophosphine) Ligands of Rh(I) Complexes: Versatile cis−trans Chelation Driven by an Interplay of Electrostatic and Orbital Effects§

The synthesis, structure, and electronic properties of a dicationic bis(amidiniophosphine) based on an o-phenylene bridge is described. In spite of the presence of two facing P-conjugated positive charges, this electron-poor diphosphine is shown to act as a versatile chelating ligand in a series of stable rhodium(I) complexes. IR analysis of a carbonyl complex showed that the sigma-donating versus pi-accepting character of the cationic ligand is comparable to that of a neutral trialkylphosphite. While the ligand is trans-chelating at a neutral Rh(I) center, it switches to cis-chelating at a cationic Rh(+) center. This and other unusual geometrical features revealed by X-ray diffraction analyses are interpreted by a subtle interplay between antisymbiotic trans preference, electrostatic repulsion, and relative Lewis acidity of the transition metal centers.

Chiral Nickel(II) and Palladium(II) NCN-Pincer Complexes Based on Substituted Benzene: Synthesis, Structure, and Lewis Acidity

Air- and moisture-stable nickel(II) and palladium(II) complexes from phebox pincer ligands 1−5 have been synthesized, and their Lewis activity was investigated. 2,6-Bis(oxazolinyl)phenylnickel halide complexes [NiX(phebox)] and 2,6-bis(oxazolinyl)phenylpalladium halide complexes [PdX(phebox)] were synthesized via oxidative addition with [Ni(COD)2] or Pd2dba3, respectively, followed by halide abstraction using silver(I) salts to form complexes such as [Ni(phebox)][ClO4] (41). Herein, X-ray crystal structures are reported for several pincer complexes having the expected square-planar geometries with the terdentate NCN pincer system. Complexes with general structure [M(phebox)]X where X is a halogen showed no relative Lewis acidity. On the other hand, complexes where X was exchanged for a less coordinating counterion showed increased Lewis acidity. The relative Lewis acidity varies depending on the substituents on the benzene core of the pincer ligands, due to the electronic effects of the ligand on the metal center.

Group 13 trihalide complexes of 9-fluorenone: a comparison of methods for assigning relative Lewis acidity

The 9-fluorenone complexes MX 3(9-fluorenone) [M=B, X=Cl ( 1); M=Al, X=Cl, Br ( 2), I ( 4), M=Ga; X=Cl ( 5), Br ( 6), I ( 7) and AlBr 3(9-fluorenone) 2 ( 3) have been prepared and characterized by NMR, IR and UV–vis spectroscopy and X-ray crystallography ( 1, 2, 3, 5 and 6). Complexes of MgCl 2 ( 8), HgCl 2 ( 9), ZrCl 4 ( 10) and SnCl 4 ( 11) were characterized by IR and UV–vis spectroscopy, while the unusual mixed vanadium(IV,V) compound, VO(Cl)(9-fluorenone) 2(H 2O)(μ-O)(VOCl 2) ( 12) has been structurally characterized. Solution ligand dissociation energies for MX 3(9-fluorenone) were determined by variable temperature 1H-NMR spectroscopy. For all the compounds, Δ S D is large and positive, as would be expected from a dissociative process. A correlation of the results for the five structurally characterized compounds MX 3(9-fluorenone) (M=B, X=Cl; M=Al, X=Cl, Br; M=Ga, X=Cl, Br) was used to examine the suitability of the following parameters to measuring Lewis acidity: IR ν CO, UV λ max, and 13C-NMR δ CO, K eq at 298 K, Δ H, Δ G, and a variety of structural parameters. Parameters determined from X-ray crystallography appear to be controlled by inter-ligand repulsion or the ionic radii of the metal atom. The change in the ν CO band upon coordination provides a reasonable ordering for a specific metal (i.e. AlI 3≈AlBr 3>AlCl 3), but does differentiate simply between 1:1 and 1:2 complexes. Unexplainably, the shift in the δ CO in the 13C-NMR appears to be the complete opposite of other trends. It appears important to differentiate, the ability of a Lewis acidic compound to bind a Lewis base from the effect the metal compound has upon that Lewis base once coordinated. In this regard, the K eq at 298 K and Δ G would appear to be the best measures of the ability of a Lewis acid to bind a specific Lewis base, while the Δ λ max for the 9- fluorenone complexes provides a good indication as to the effect of a Lewis acid on a particular Lewis base.